aarch64: Fix bootstrap with old binutils [PR93053]

As reported in the PR, GCC 10 (and also 9.3.1 but not 9.3.0) fails to build
when using older binutils which lack LSE support, because those instructions
are used in libgcc.
Thanks to Kyrylo's hint, the following patches (hopefully) allow it to build
even with older binutils by using .inst directive if LSE support isn't
available in the assembler.

2020-04-15  Jakub Jelinek  <jakub@redhat.com>

	PR target/93053
	* configure.ac (LIBGCC_CHECK_AS_LSE): Add HAVE_AS_LSE checking.
	* config/aarch64/lse.S: Include auto-target.h, if HAVE_AS_LSE
	is not defined, use just .arch armv8-a.
	(B, M, N, OPN): Define.
	(COMMENT): New .macro.
	(CAS, CASP, SWP, LDOP): Use .inst directive if HAVE_AS_LSE is not
	defined.  Otherwise, move the operands right after the glue? and
	comment out operands where the macros are used.
	* configure: Regenerated.
	* config.in: Regenerated.