PR21166: Validate rdrand/rdseed support separately in gdb.reverse/insn-reverse-x86.c

PR21166: Validate rdrand/rdseed support separately in gdb.reverse/insn-reverse-x86.c

As reported in PR21166, there are Intel processors out there that support
rdrand but not rdseed. The fix is to verify both features separately and only
run rdrand/rdseed tests if supported.

gdb/testsuite/ChangeLog:
2017-02-23  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.reverse/insn-reverse.x86.c (check_rdrand_support): Renamed to ...
	(check_supported_features): ... this. Changed return type to void.
	(supports_rdseed): New static global.
	(rdseed): Check supports_rdseed.
	(initialize): Call check_supported_features.