[AArch64] MachO large code-model: Materialize FP constants in code.

In the large code model we have to first get the address of the GOT entry, load
the address of the constant, and then load the constant itself.

To avoid these loads and the GOT entry alltogether this commit changes the way
how FP constants are materialized in the large code model. The constats are now
materialized in a GPR and then bitconverted/moved into the FPR.

Reviewed by Tim Northover

Fixes rdar://problem/16572564.