The StackSlot operator allows to allocate a spill slot on the stack. We
are going to use this operator to pass floats through pointers to c
functions, which we need for floating point rounding in the case where
the architecture does not provide rounding instructions.

R=titzer@chromium.org, v8-arm-ports@googlegroups.com, v8-ppc-ports@googlegroups.com, v8-mips-ports@googlegroups.com

Review URL: https://codereview.chromium.org/1645653002

Cr-Commit-Position: refs/heads/master@{#33600}