During a C call, a previous value of the stack pointer is stored in a
platform specific callee saved register. Loading the out argument of the
C call might overwrite the value in that register, if the destination
register collides with the platform specific register. Hence, do first
use that register to restore the previous stack pointer, and only then
load the out argument.
Similarly, when pushing arguments to the stack, do first push all
values and then set the platform specific register in order to avoid
overwriting an argument value held in that register.

Drive-by: Fix offset computations for parameters pushed to the stack
for c calls.

R=titzer@chromium.org

Bug: chromium:820802,chromium:820896,chromium:820807,v8:6600
Change-Id: If4567467b7912454f0bd2cad5927233c98894b03
Reviewed-on: https://chromium-review.googlesource.com/959064Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51916}