shared/architecture.h: fix preprocessor directives

Use 'if defined()', not 'ifdef defined()'. Fixes the following warning.

  CC       src/shared/architecture.lo
In file included from src/shared/architecture.c:24:0:
src/shared/architecture.h:89:17: warning: extra tokens at end of #ifdef
directive [enabled by default]
 #  ifdef defined(WORDS_BIGENDIAN)
                 ^