rs6000: Skip empty inline asm in rs6000_update_ipa_fn_target_info [PR111366]

rs6000: Skip empty inline asm in rs6000_update_ipa_fn_target_info [PR111366]

PR111366 exposes one thing that can be improved in function
rs6000_update_ipa_fn_target_info is to skip the given empty
inline asm string, since it's impossible to adopt any
hardware features (so far HTM).

Since this rs6000_update_ipa_fn_target_info related approach
exists in GCC12 and later, the affected project highway has
updated its target pragma with ",htm", see the link:
https://github.com/google/highway/commit/15e63d61eb535f478bc
I'd not bother to consider an inline asm parser for now but
will file a separated PR for further enhancement.

	PR target/111366

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_update_ipa_fn_target_info): Skip
	empty inline asm.

gcc/testsuite/ChangeLog:

	* g++.target/powerpc/pr111366.C: New test.