Remove some extraneous whitespace.
Reformat this routine slightly.
Revert part of a fix from 2010, changes since then:

a) x86-64 TLS has been documented
b) the code path should use movq for the correct relocation
   to be generated.

I've also added a fixme for the test case that we should improve
the code generated, it should look something like is documented
in the tls abi document.