Fix up x86_64_sel4 arch after merge

Support a seL4 x86_64 build

change jmp_buf to share an underlying type and struct tag with sigjmp_buf

this is necessary to meet the C++ ABI target. alternatives were
considered to avoid the size increase for non-sig jmp_buf objects, but
they seemed to have worse properties. moreover, the relative size
increase is only extreme on x86[_64]; one way of interpreting this is
that, if the size increase from this patch makes jmp_buf use too much
memory, then the program was already using too much memory when built
for non-x86 archs.

jmp_buf overhaul fixing several issues

on arm, the location of the saved-signal-mask flag and mask were off
by one between sigsetjmp and siglongjmp, causing incorrect behavior
restoring the signal mask. this is because the siglongjmp code assumed
an extra slot was in the non-sig jmp_buf for the flag, but arm did not
have this. now, the extra slot is removed for all archs since it was
useless.

also, arm eabi requires jmp_buf to have 8-byte alignment. we achieve
that using long long as the type rather than with non-portable gcc
attribute tags.

Port musl to x86-64. One giant commit!