AArch64: Enable early scheduling for -O3 and higher (PR118351)

AArch64: Enable early scheduling for -O3 and higher (PR118351)

Enable the early scheduler on AArch64 for O3/Ofast.  This means GCC15 benefits
from much faster build times with -O2, but avoids the regressions in lbm which
is very sensitive to minor scheduling changes due to long FMA chains.

gcc:
	PR target/118351
	PR other/38768
	* common/config/aarch64/aarch64-common.cc: Enable early scheduling with
	-O3 and higher.
	* doc/invoke.texi (-fschedule-insns): Update comment.