Set ELF_OSABI for x86 and sparc

The idea of this patch is to match the solaris target over other
targets if e_ident contains ELFOSABI_SOLARIS.  The solaris target will
continue to recognise ELFOSABI_NONE objects.

This has the side effect of disabling gnu features that require
ELFOSABI_GNU, such as ifuncs.  I think that is correct, so I've made
the required testsuite changes to fix the resulting regressions:
FAIL: nm --ifunc-chars (assembly)
FAIL: mbind sections without SHF_ALLOC

The patch also sets ELF_OSABI for the gnu x86 and sparc targets,
for the same reason as the solaris targets.  This doesn't mean object
files will automatically be marked ELFOSABI_GNU/LINUX.  As before that
will only happen when certain GNU extensions are present.

bfd/
	* elf32-i386.c: Define ELF_OSABI for solaris and gnu targets.
	* elf32-sparc.c: Likewise.
	* elf64-sparc.c: Likewise.
	* elf64-x86-64.c: Likewise.
	* format.c (bfd_check_format_matches): Bump match_priority
	for matching e_ident EI_OSABI.
binutils/
	* testsuite/binutils-all/nm.exp: Use !supports_gnu_osabi to
	disable ifunc test.
gas/
	* testsuite/gas/elf/section13.d: Only run on supports_gnu_osabi
	targets.  Remove xfails.