[JITLink][AArch64] Use R-X permissions for the GOT.

This consistent with ld64's treatment of the GOT, but the main aim here is a
short-term workaround for a bad interaction between stub code sequences and
memory layout: Stubs use LDRLiteral19 relocations to reference the GOT, but
BasicLayout currently puts RW- segments between R-- and R-X segments -- a large
RW- segment (or a large R-- for that matter) can cause the relocation to fail
with an out-of-range error.

Putting the GOT in R-X fixes this efficiently in practice. A more robust fix
will be to use a longer code sequence to materialize the GOT pointer and then
rewrite the stub to use a shorter sequence where possible.