preprocessor: c++: Support `#pragma GCC target' macros [PR87299]

preprocessor: c++: Support `#pragma GCC target' macros [PR87299]

`#pragma GCC target' is not currently handled in preprocess-only mode (e.g.,
when running gcc -E or gcc -save-temps). As noted in the PR, this means that
if the target pragma defines any macros, those macros are not effective in
preprocess-only mode. Similarly, such macros are not effective when
compiling with C++ (even when compiling without -save-temps), because C++
does not process the pragma until after all tokens have been obtained from
libcpp, at which point it is too late for macro expansion to take place.

Since r13-1544 and r14-2893, there is a general mechanism to handle pragmas
under these conditions as well, so resolve the PR by using the new "early
pragma" support.

toplev.cc required some changes because the target-specific handlers for
`#pragma GCC target' may call target_reinit(), and toplev.cc was not expecting
that function to be called in preprocess-only mode.

I added some additional testcases from the PR for x86. The other targets
that support `#pragma GCC target' (aarch64, arm, nios2, powerpc, s390)
already had tests verifying that the pragma sets macros as expected; here I
have added -save-temps versions of some of them, to test that they now work
in preprocess-only mode as well.

gcc/c-family/ChangeLog:

	PR preprocessor/87299
	* c-pragma.cc (init_pragma): Register `#pragma GCC target' and
	related pragmas in preprocess-only mode, and enable early handling.
	(c_reset_target_pragmas): New function refactoring code from...
	(handle_pragma_reset_options): ...here.
	* c-pragma.h (c_reset_target_pragmas): Declare.

gcc/cp/ChangeLog:

	PR preprocessor/87299
	* parser.cc (cp_lexer_new_main): Call c_reset_target_pragmas ()
	after preprocessing is complete, before starting compilation.

gcc/ChangeLog:

	PR preprocessor/87299
	* toplev.cc (no_backend): New static global.
	(finalize): Remove argument no_backend, which is now a
	static global.
	(process_options): Likewise.
	(do_compile): Likewise.
	(target_reinit): Don't do anything in preprocess-only mode.
	(toplev::main): Adapt to no_backend change.
	(toplev::finalize): Likewise.

gcc/testsuite/ChangeLog:

	PR preprocessor/87299
	* c-c++-common/pragma-target-1.c: New test.
	* c-c++-common/pragma-target-2.c: New test.
	* g++.target/i386/pr87299-1.C: New test.
	* g++.target/i386/pr87299-2.C: New test.
	* gcc.target/i386/pr87299-1.c: New test.
	* gcc.target/i386/pr87299-2.c: New test.
	* gcc.target/s390/target-attribute/tattr-2b.c: New test.
	* gcc.target/aarch64/pragma_cpp_predefs_1b.c: New test.
	* gcc.target/arm/pragma_arch_attribute_1b.c: New test.
	* gcc.target/nios2/custom-fp-2b.c: New test.
	* gcc.target/powerpc/float128-3b.c: New test.