Remove obsolete Tru64 UNIX V5.1B support

[Bruce Korb]

CF:
        fixincludes:
       * inclhack.def (alpha___extern_prefix): Remove.
       (alpha___extern_prefix_standards): Remove.
       (alpha___extern_prefix_sys_stat): Remove.
       (alpha_bad_lval): Remove.
       (alpha_pthread): Remove.
       (alpha_pthread_gcc): Remove.
       (alpha_pthread_init): Remove.
       * fixincl.x: Regenerate.
       * tests/base/pthread.h [ALPHA_PTHREAD_CHECK]: Remove.
       [ALPHA_PTHREAD_GCC_CHECK]: Remove.
       [ALPHA_PTHREAD_INIT_CHECK]: Remove.
       * tests/base/standards.h: Remove.
       * tests/base/sys/stat.h [ALPHA___EXTERN_PREFIX_SYS_STAT_CHECK]:
       Remove.
       * tests/base/testing.h [ALPHA___EXTERN_PREFIX_CHECK]: Remove.
       [ALPHA_BAD_LVAL_CHECK]: Remove.

Seems reasonable to me...