Export _mmap and __sys_mmap from libc.so

Unlike the other syscalls these two symbols were missing from the
version script. I noticed this while looking into the compiler-rt
runtime libraries for CHERI.

Reviewed by: brooks
Obtained from: https://github.com/CTSRD-CHERI/cheribsd/pull/1063
MFC after: 3 days

(cherry picked from commit 395db99f32bc615a3df2cd469e9537938d022c88)