TLS and networking fixes

The series fixes up TLS storage (in the easiest possible way) for x86-64, and adds several important networking syscalls. `rr record firefox` now works!

I've tried recording bits of mochitests, but I run into asserts in `AddressSpace.cc`.  Unfortunately, I'm not getting any logging output from `AddressSpace.cc` at the moment (`AddressSpace::dump` doesn't seem to work), so I'm not entirely sure what's going wrong yet.