Add support for Hexagon

Add support for Hexagon

This implements initial support for the Hexagon architecture. The
Hexagon ABI spec can be seen at
https://lists.llvm.org/pipermail/llvm-dev/attachments/20190916/21516a52/attachment-0001.pdf

A hello_hexagon.ko test is also added.

$ head tests/test-suite.log
  [...]
  # TOTAL: 275
  # PASS:  269
  # SKIP:  6
  # XFAIL: 0
  # FAIL:  0
  # XPASS: 0
  # ERROR: 0

$ cat tests/run-strip-reloc-ko.sh.log
  [...]
  runtest hello_hexagon.ko
  PASS run-strip-reloc-ko.sh (exit status: 0)

	* backends/Makefile.am (modules): Add hexagon.
	(hexagon_SRCS): New var for hexagon_init.c and hexagon_symbol.c.
	(libebl_backends_a_SOURCES): Add hexagon_SRCS.
	* backends/hexagon_init.c: New file.
	* backends/hexagon_reloc.def: Likewise.
	* backends/hexagon_symbol.c: Likewise.
	* libebl/eblopenbackend.c (hexagon_init): Declare.
	(machines): Add hexagon.
	* libelf/elf-knowledge.h: Add hexagon e_flags values, section
        indices and and relocs.
	* src/elflint.c (valid_e_machine): Add EM_QDSP6.
	* tests/Makefile.am (EXTRA_DIST): Add hello_hexagon.ko.bz2.
	* tests/hello_hexagon.ko.bz2: New test file.
	* tests/run-strip-reloc-ko.sh: Add hello_hexagon.ko.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>