[abi] jmp_buf size ABI transition, phase 1

This is the first step in a soft transition to change the size of
the jmp_buf type in the AArch64 Zircon libc ABI.  In phase 1,
libc-tests will accept either the old or new size.  In phase 2,
libc will change its ABI.  In phase 3, libc-tests will accept
only the new size.

Bug: ZX-1947 #comment begin soft transition for jmp_buf ABI
Change-Id: I551d51c452bdf777be543a0db5e070aaff8f86a7