thread-local storage for amd64_apple

It is quite similar to arm64_apple.
Probably, the call that needs to be
generated also provides extra
invariants on top of the regular
abi, but I have not checked that.

Clang generates code that is a bit
neater than qbe's because, on x86,
a load can be fused in a call
instruction! We do not bother with
supporting these since we expect
only sporadic use of the feature.

For reference, here is what clang
might output for a store to the
second entry of a thread-local
array of ints:

        movq    _x@TLVP(%rip), %rdi
        callq   *(%rdi)
        movl    %ecx, 4(%rax)

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