Remove unnecessary padding in ucontext_t

- removed extra padding from ucontex_t on x86, x86_64 and mips
- force ucontext_t struct to use 64 bits for the sigmask on all arches
(previsouly wrong since sigset_t on arm and x86 is only 32 bits)
- arm and arm64 continue to use padding to match glibc sigset_t

This brings Bionic on par with the kernel w.r.t ucontext padding.

Bug: 12828904
Change-Id: Ia8915ace694ecb4695603a334e697985f4c3e7aa