Fix a bug in __builtin_unreachable configure check
Fix a bug in __builtin_unreachable configure check

In 1167e9e, I accidentally tested je_cv_gcc_builtin_ffsl instead of
je_cv_gcc_builtin_unreachable (copy-paste error), which meant that
JEMALLOC_INTERNAL_UNREACHABLE was always getting defined as abort even if
__builtin_unreachable support was detected.

8l/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,8l/jemalloc,wqfish/jemalloc,wqfish/jemalloc,8l/jemalloc,8l/jemalloc