Add support for RISC-V

Add support for RISC-V

This implements initial support for the RISC-V architecture.  It has
been tested with qemu linux-user emulation
<https://build.opensuse.org/package/live_build_log/openSUSE:Factory:RISCV/elfutils/standard/riscv64>,
with the following unresolved issues

FAIL: run-strip-strmerge.sh
===========================

elflint /home/abuild/rpmbuild/BUILD/elfutils-0.170/tests/elfstrmerge
section [32] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x51c0 does not match .got section address 0x5010
section [32] '.symtab': symbol 119: st_value out of bounds
FAIL run-strip-strmerge.sh (exit status: 1)

FAIL: run-elflint-self.sh
=========================

section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x6220 does not match .got section address 0x6008
section [33] '.symtab': symbol 135: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/addr2line
section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x91d8 does not match .got section address 0x9040
section [33] '.symtab': symbol 163: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elfcmp
section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x1a2a0 does not match .got section address 0x1a040
section [33] '.symtab': symbol 267: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint
section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0xd418 does not match .got section address 0xd0f0
section [33] '.symtab': symbol 238: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/nm
section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0xa2d0 does not match .got section address 0xa0b0
section [33] '.symtab': symbol 193: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/objdump
section [33] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x2e5b8 does not match .got section address 0x2e058
section [33] '.symtab': symbol 410: st_value out of bounds
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/readelf
section [32] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x171b8 does not match .got section address 0x17010
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/libelf/libelf.so
section [32] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0x3a5b0 does not match .got section address 0x3a0d8
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/libdw/libdw.so
section [31] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0xd0c8 does not match .got section address 0xd008
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/backends/libebl_i386.so
section [31] '.symtab': _GLOBAL_OFFSET_TABLE_ symbol value 0xf0c8 does not match .got section address 0xf008
*** failure in /home/abuild/rpmbuild/BUILD/elfutils-0.170/src/elflint --quiet --gnu-ld /home/abuild/rpmbuild/BUILD/elfutils-0.170/backends/libebl_x86_64.so
FAIL run-elflint-self.sh (exit status: 1)

FAIL: run-native-test.sh
========================

/home/abuild/rpmbuild/BUILD/elfutils-0.170/tests/allregs: dwfl_module_register_names: no backend registers known
FAIL run-native-test.sh (exit status: 1)

FAIL: run-backtrace-native-core.sh
==================================

backtrace: backtrace.c:111: callback_verify: Assertion `symname && strcmp (symname, "raise") == 0' failed.
./test-subr.sh: line 84: 26040 Aborted                 (core dumped) LD_LIBRARY_PATH="${built_library_path}${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH" $VALGRIND_CMD "$@"
backtrace-child-core.26027: no main
rmdir: failed to remove 'test-26016': Directory not empty
FAIL run-backtrace-native-core.sh (exit status: 1)

In addition, all tests that use ptrace are failing as linux-user
emulation does not implement it.

There are no regressions for a x86_64 build.

Signed-off-by: Andreas Schwab <schwab@suse.de>