Refactor PUSH/POP in ARM assemblers.

Refactor PUSH/POP in ARM assemblers.

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 .