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.

llvm-svn: 197711