MIPS/LD/testsuite: mips-elf-flags: Use `-h' only with `readelf'

For the purpose of link-time object compatibility handling verification
code in the `good_combination' procedure from the `mips-elf-flags.exp'
test script only examines the `e_flags' member of the ELF file header
and ignores data from any ELF program or section headers present.  Use
`readelf -h' rather than `readelf --headers' then to obtain data for
examination, avoiding unnecessary processing to extract this extraneous
information.

	ld/
	* testsuite/ld-mips-elf/mips-elf-flags.exp (good_combination):
	Use `readelf -h' rather than `readelf --headers'.