sysdeps/{i386, x86_64}/mempcpy_chk.S: fix linknamespace for __mempcpy_chk

sysdeps/{i386, x86_64}/mempcpy_chk.S: fix linknamespace for __mempcpy_chk

On i386 and x86_64, for libc.a specifically, __mempcpy_chk calls
mempcpy which leads POSIX routines to call non-POSIX mempcpy indirectly.

This leads the linknamespace test to fail when glibc is built with
__FORTIFY_SOURCE=3.

Since calling mempcpy doesn't bring any benefit for libc.a, directly
call __mempcpy instead.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>