<regex.h>: reserve a regcomp field for REG_POSIX

For libc regcomp, this will be a nop. libregex will take this to mean that
it needs to turn off GNU extensions, effectively switching it back to the
POSIX-compliant libc implementation at runtime.

(cherry picked from commit 7c5ec5fe6afb50ba5c83ad0b3dab036f91b7dafe)