SIGSTKSZ/MINSIGSTKSZ too small on 64bit

[Anton Blanchard]

Hi,

Alan has been looking at a glibc test fail. His analysis shows SEGVs
in signal handlers using sigaltstack, and that MINSIGSTKSZ and SIGSTKSZ
are too small.

We increased the size of rt_sigframe in commit 2b0a576d15e0
(powerpc: Add new transactional memory state to the signal context) but
didn't bump either SIGSTKSZ and MINSIGSTKSZ. We need to do that in both
the kernel and glibc, but I'm a bit worried we could have broken
existing applications that use sigaltstack.

Anton