AArch64: remove ls64 from being mandatory on armv8.7-a..

AArch64: remove ls64 from being mandatory on armv8.7-a..

The Arm Architectural Reference Manual (Version J.a, section A2.9 on FEAT_LS64)
shows that ls64 is an optional extensions and should not be enabled by default
for Armv8.7-a.

This drops it from the mandatory bits for the architecture and brings GCC inline
with LLVM and the achitecture.

Note that we will not be changing binutils to preserve compatibility with older
released compilers.

gcc/ChangeLog:

	* config/aarch64/aarch64-arches.def (AARCH64_ARCH): Remove LS64 from
	Armv8.7-a.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/acle/ls64.C: Add +ls64.
	* g++.target/aarch64/acle/ls64_lto.C: Likewise.
	* gcc.target/aarch64/acle/ls64_lto.c: Likewise.
	* gcc.target/aarch64/acle/pr110100.c: Likewise.
	* gcc.target/aarch64/acle/pr110132.c: Likewise.
	* gcc.target/aarch64/pragma_cpp_predefs_2.c: Correct header checks.