rs6000: Fix up build of non-glibc/aix/darwin powerpc* targets [PR104298]

rs6000: Fix up build of non-glibc/aix/darwin powerpc* targets [PR104298]

As reported by Martin, while David has added OPTION_GLIBC define to aix
and Iain to darwin, all the other non-linux targets now fail because
rs6000.md macro isn't defined.

One possibility is to define this macro in option-defaults.h which on rs6000
targets is included last, then we don't need to define it in aix/darwin
headers and for targets using linux.h or linux64.h it will DTRT too.

The other option is the first 2 hunks + changing the 3
   if (!OPTION_GLIBC)
     FAIL;
cases in rs6000.md to e.g.
 #ifdef OPTION_GLIBC
   if (!OPTION_GLIBC)
 #endif
     FAIL;
or to:
 #ifdef OPTION_GLIBC
   if (!OPTION_GLIBC)
 #else
   if (true)
 #endif
     FAIL;
(the latter case if Richi wants to push the -Wunreachable-code changes for
GCC 13).

2022-01-31  Jakub Jelinek  <jakub@redhat.com>

	PR target/104298
	* config/rs6000/aix.h (OPTION_GLIBC): Remove.
	* config/rs6000/darwin.h (OPTION_GLIBC): Likewise.
	* config/rs6000/option-defaults.h (OPTION_GLIBC): Define to 0
	if not already defined.