This should also save and restore non-volatile Altivec registers, but that
needs to wait on solving two problems:
1. Adding the nonvolatile vector registers means we need 5 more than _JBLEN
entries in jmp_buf on 32-bit targets (64-bit is OK).
2. Need to figure out how to determine if saving/restoring vector regs
is supported on the current CPU from userland.
MFC after: 1 month
usermode context switches (long jumps and ucontext operations). If these
are used across threads, multiple threads can end up with the same TLS base.
Madness will then result.
This makes behavior on PPC match that on x86 systems and on Linux.
MFC after: 10 days
