ARM: fix literal load with positive offset encoding

When using a positive offset, literal loads where encoded
as if it was negative, because:
- The sign bit was not assigned to an operand
- The addrmode_imm12 operand was not encoding the sign bit correctly

This patch also makes the assembler look at the .w/.n specifier for
loads.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184182 91177308-0d34-0410-b5e6-96231b3b80d8