Implement the .ltorg directive for ARM assembly

Implement the .ltorg directive for ARM assembly

This directive will write out the assembler-maintained constant
pool for the current section. These constant pools are created to
support the ldr-pseudo instruction (e.g. ldr r0, =val).

The directive can be used by the programmer to place the constant
pool in a location that can be reached by a pc-relative offset in
the ldr instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197711 91177308-0d34-0410-b5e6-96231b3b80d8