regex: simplify Gnulib port

This simplifies the code, by removing stuff intended for porting
to Gnulib but no longer needed there.
* posix/regcomp.c [!_LIBC]: No need to put #ifdef _LIBC around
uses of libc_hidden_def, weak_alias.
* posix/regcomp.c, posix/regexec.c: Use __restrict rather than
_Restrict_ except for public-facing headers.
* posix/regex_internal.h (attribute_hidden) [!_LIBC]:
Remove; already defined elsewhere.
* posix/regex.c, posix/regex_internal.h:
Use __GNUC_PREREQ instead of rolling our own.
* posix/regex_internal.h (__GNUC_PREREQ): Remove duplicate defn.