This commit adds record full support for rv64gc instruction set

This commit adds record full support for rv64gc instruction set

It includes changes to the following files:
- gdb/riscv-linux-tdep.c, gdb/riscv-linux-tdep.h: adds facilities to record
syscalls.
- gdb/riscv-tdep.c, gdb/riscv-tdep.h: adds facilities to record execution of
rv64gc instructions.
- gdb/configure.tgt: adds new files for compilation.
- gdb/testsuite/lib/gdb.exp: enables testing of full record mode for RISC-V
targets.
- gdb/syscalls/riscv-canonicalize-syscall-gen.py: a script to generate
function that canonicalizes RISC-V syscall. This script can simplify support
for syscalls on rv32 and rv64 system (currently support only for rv64).  To
use this script you need to pass a path to a file with syscalls description
from riscv-glibc (example is in the help message). The script produces a
mapping from syscall names to gdb_syscall enum.
- gdb/riscv-canonicalize-syscall.c: the file generated by the previous script.
- gdb/doc/gdb.texinfo: notification that record mode is enabled in RISC-V.
- gdb/NEWS: notification of new functionality.

Approved-By: Guinevere Larsen <guinevere@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>