MC: support different sized constants in constant pools

On AArch64 the pseudo instruction ldr <reg>, =... supports both
32-bit and 64-bit constants. Add support for 64 bit constants for
the pools to support the pseudo instruction fully.

Changes the AArch64 ldr-pseudo tests to use 32-bit registers and
adds tests with 64-bit registers.

Patch by Janne Grunau!

Differential Revision: http://reviews.llvm.org/D4279