Switch from USE_GAS/GLD to HAVE_GNU_AS/LD

Switch from USE_GAS/GLD to HAVE_GNU_AS/LD

We currently have two ways to distinguish GNU and non-GNU assemblers and
linkers:

* USE_GAS and USE_GLD, defined via gcc/config/usegas.h and usegld.h
  which are included via config.gcc

* HAVE_GNU_AS and HAVE_GNU_LD, determined from gcc/configure.ac

This is confusing and leads to weird mistakes like combining both into
an undefined USE_GNU_LD.  Since both are based on the same info ($gas
resp. $gnu_ld in gcc/configure.ac), it seems best to standardise on one
of them.  Since the USE_* form is almost exclusively used in
Solaris-specific contexts, it's clearer to use the HAVE_GNU_* forms
instead.

This is what this patch does.

Most of the changes are either Solaris-specific or border on obvious.

Outside of Solaris code, there are only two cases:

* gcc/config/ia64/hpux.h has one macro guarded by !USE_GAS.  This wasn't
  defined before since the configuration for this target doesn't include
  usegas.h.  However, the code is still needed, so the HAVE_GNU_AS form is
  now used.

* Uses of usegas.h in various powerpc*-*-* configurations in
  gcc/config.gcc.  Those make no difference since USE_GAS isn't used in any
  of the affected files.

Bootstrapped without regressions on i386-pc-solaris2.11 and
sparc-sun-solaris2.11 (as/ld, gas/ld, and gas/gld configurations).

2025-11-18  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* config/i386/sol2.h: Replace USE_GAS by HAVE_GNU_AS.
	Replace USE_GLD by HAVE_GNU_LD.
	* config/sol2.h: Likewise.
	* config/sparc/sol2.h: Likewise.
	* config/i386/i386.cc (i386_solaris_elf_named_section)
	[TARGET_SOLARIS]: Replace USE_GAS by HAVE_GNU_AS.
	* config/ia64/hpux.h: Likewise.

	* config.gcc: Remove usegas.h, usegld.h.
	* config/usegas.h: Remove.
	* config/usegld.h: Remove

	gcc/go:
	* gospec.cc (lang_specific_driver) [TARGET_SOLARIS]: Replace
	USE_GLD by HAVE_GNU_LD.