posix: Sync regex with gnulib

It sync with gnulib commit 6cfb4302b3e1da14d706198b693558290e9b00f4
and contains the fixes:

https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=32915b2a8a43825720755113bdffe9f67a591748
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=48f07576b8cd935b48e1050551f45ab1a79b9f01
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=5e407aba1f775d51b25481cb55f324c9868f62d7
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=4e02b30c761c76d04057fa5f6bba71401f9310cd
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=79f8ee4e389f8cb1339f8abed9a7d29816e2a2d4

Checked on x86_64-linux-gnu and i686-linux-gnu.