Align IRIX/OSF1_THREADS definition in gc_config_macros.h with gcconfig.h

Align IRIX/OSF1_THREADS definition in gc_config_macros.h with gcconfig.h

The condition of defining GC_IRIX_THREADS and GC_OSF1_THREADS
(if GC_THREADS) in gc_config_macros.h is now the same as that of
IRIX5 and OSF1 macros in gcconfig.h, respectively.

* include/gc_config_macros.h [GC_THREADS] (GC_HPUX_THREADS,
GC_SOLARIS_THREADS): Replace !defined(__linux__) with "#elif".
* include/gc_config_macros.h [GC_THREADS] (GC_OPENBSD_THREADS,
GC_FREEBSD_THREADS, GC_NETBSD_THREADS): Handle the case before
GC_OSF1_THREADS, GC_IRIX_THREADS, GC_SOLARIS_THREADS, GC_DARWIN_THREADS;
remove FIXME.
* include/gc_config_macros.h [GC_THREADS] (GC_IRIX_THREADS): Check
mips and _mips predefined macros in addition to __mips; do not define
if any of [_]nec_ews, [__]ultrix predefined macro is defined.