Refactors methods emit() and emitIAS() of InstARM32Push and InstARM32Pop
to separate out the selection of assembler instructions from instruction
emission, using template methods.

Template method assemble() provides a single implementation for emit() and emitIAS(). This method calls template functions in the assembler to generate textual and binary forms of the instruction.

BUG= None
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1535233002 .