[libc++][lit][AIX] Enable test case last_write_time.pass.cpp for AIX

Summary:
This patch enables libc++ LIT test case last_write_time.pass.cpp for AIX. Because system call utimensat() of AIX which is used in the libc++ implementation of last_write_time() does not accept the times parameter with a negative tv_sec or tv_nsec field, testing of setting file time to before epoch time is excluded for AIX.

Reviewed by: ldionne, libc++

Differential Revision: https://reviews.llvm.org/D133124