Fix pre-L MB_CUR_MAX.

The default locale is POSIX, not C.UTF-8. POSIX explicitly states
that MB_CUR_MAX is 1 for the POSIX locale:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdlib.h.html

Test: Made change in the NDK, built libc++ against it, ran NDK libc++
      tests.
Bug: None

Change-Id: Ic2f6f96aa4a7f20d619030f41323831d01002715