[PATCH] Implement Named Register Global Variables in LLVM

[Rafael Ávila de Espíndola]

+Warning: There is no register reservation at the moment, and so far it
+only works with the stack pointer on selected architectures (ARM, ARM64,
+x86_64 and AArch64).

This is still inaccurate. There are two restrictions that we should mention:

* There is no reservation and we error on allocatable registers. That
is not "for now" or "at the moment".
* The only thing that is currently implemented is the stack registers
in some architectures, but we hope to expand that to other
non-allocatable registers in the future.

http://reviews.llvm.org/D3261