MIPS/LD/testsuite: mips-elf-flags: Use `remote_exec' to call `readelf'

For ELF file header flag verification done in `good_combination' in the
`mips-elf-flags.exp' test script the version of `readelf' built along
with the rest of binutils has to be used rather than any such executable
already present on the build system, so that flags recognized by the
tool match those supported by LD being tested.  Use `remote_exec' as
elsewhere in the LD test framework and also with GAS and LD used here,
getting and arranging for extra reporting of `readelf' calls included in
test logs on this occasion as well.

	ld/
	* testsuite/ld-mips-elf/mips-elf-flags.exp (good_combination):
	Use `remote_exec' to call `readelf'.  Log the command issued.