AArch64: Have empty HWCAPs string ignored during native feature detection

AArch64: Have empty HWCAPs string ignored during native feature detection

This patch makes the feature detection code for AArch64 GCC not add features
automatically when the feature had no hwcaps string to match against.

This means that -mcpu=native no longer adds feature flags such as +profile.
The behavior wasn't noticed before because at the time +profile was added a bug
was preventing any feature bits from being added by native detections.

The loop has also been changed as Jakub specified in order to avoid a memory
leak that was present in the existing code and to be slightly more efficient.

gcc/ChangeLog:

	PR target/88530
	* config/aarch64/aarch64-option-extensions.def: Document it.
	* config/aarch64/driver-aarch64.c (host_detect_local_cpu): Skip feature
	if empty hwcaps.

gcc/testsuite/ChangeLog:

	PR target/88530
	* gcc.target/aarch64/options_set_10.c: New test.

From-SVN: r269276