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
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