libc: Fix the definition of SIGRTMAX

After this change, SIGRTMAX will be set to 64 (instead of 32 currently).
Note that this doesn't change the fact that our sigset_t is still defined
as a 32-bit unsigned integer, so most functions that deal with this type
won't support real-time signals though.

Change-Id: Ie1e2f97d646f1664f05a0ac9cac4a43278c3cfa8
3 files changed