regex: __builtin_expect → __glibc_unlikely

[BZ#23744]
This refactoring was prompted by a problem when the regex code is
used as part of Gnulib and when the builder’s compiler does not grok
__builtin_expect.  Problem reported for Gawk by Nelson H.F. Beebe in:
https://lists.gnu.org/r/bug-gnulib/2018-09/msg00137.html
Although this refactoring does not fix the problem directly,
we might as well have Gawk use the now-preferred glibc style for when
__builtin_expect is unavailable.
* posix/regex_internal.h (BE): Remove.
All uses replaced by __glibc_unlikely or __glibc_likely.