MIPS/LD/testsuite: mips-elf-flags: Add MIPS ABI Flags handling

Complement commit 351cdf24 ("[MIPS] Implement O32 FPXX, FP64 and
FP64A ABI extensions") and add optional MIPS ABI Flags handling to
`good_combination' in the `mips-elf-flags.exp' test script.  This lets
callers of this procedure request to examine MIPS ABI Flags in addition
to the `e_flags' member of the ELF file header so as to verify that
flags are merged correctly by LD.  The presence of further arguments
triggers this verification, in which case `readelf' is called with the
`-A' option additionally specified and the ISA member, the ISA Extension
member and the ASEs member will be examined as per the arguments.

Unlike with `readelf -h' output consider a failure to retrieve the
member requested a test case failure rather than an unresolved result.
This is because unlike with the `e_flags' member of the ELF file header
which is always there in any valid ELF file the MIPS ABI Flags structure
is optional in LD output and the absence of this structure when expected
is surely a bug in LD.

	ld/
	* testsuite/ld-mips-elf/mips-elf-flags.exp (good_combination):
	Add an `args' final argument and examination code for `readelf
	-A' output.  Update procedure description accordingly.