Fix Solaris/sparc detection in case of strict C compliance is enforced

Fix Solaris/sparc detection in case of strict C compliance is enforced

* include/gc_config_macros.h [__i386__ || __amd64__] (GC_SOLARIS_THREADS):
Define also if __sun is defined (treat __sun as a synonym to sun).
* include/private/gcconfig.h [__i386__ || __amd64] (SOLARIS):
Likewise.
* include/gc_config_macros.h [sun] (GC_SOLARIS_THREADS): Define also
if __amd64 is defined (treat __amd64 as a synonym to __amd64__).
* include/gc_config_macros.h [sparc && unix] (DRSNX): Do not define if
SPARC and SOLARIS are already defined.