libc: regex: partial revert of r368358

libc: regex: partial revert of r368358

Part of the libregex functionality leaked into the tests it shares with
the standard regex(3). Introduce a P flag to set the REG_POSIX cflag to
indicate that libc regex should effectively do nothing while libregex should
specifically run it in non-extended mode.

This unbreaks the libc/regex test run.

Reported by:	Jenkins