Support for constant islands in the ARM JIT.

Support for constant islands in the ARM JIT.

Since the ARM constant pool handling supercedes the standard LLVM constant
pool entirely, the JIT emitter does not allocate space for the constants,
nor initialize the memory. The constant pool is considered part of the
instruction stream.

Likewise, when resolving relocations into the constant pool, a hook into
the target back end is used to resolve from the constant ID# to the
address where the constant is stored.

For now, the support in the ARM emitter is limited to 32-bit integer. Future
patches will expand this to the full range of constants necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58338 91177308-0d34-0410-b5e6-96231b3b80d8