Some fixes for x86-64 JIT. Make it use small code

model, except for external calls; this makes
addressing modes PC-relative.  Incomplete.

The assertion at the top of Emitter::runOnMachineFunction
was obviously bogus (always true) so I removed it.
If someone knows what the correct test should be to cover
all the various targets, please fix.

llvm-svn: 54656