MIPS: Fix flaky mjsunit/harmony/generators-iteration test failure.

This fixes the result allocation in EmitCreateIteratorResult,
when a GC is required. The Runtime call returns the allocated
pointer in v0 register, but the rest of the function was
expected it in a0 register. Refactored the function to use v0 for
the result in the whole function.

BUG=

Review URL: https://codereview.chromium.org/21244003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15965 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Introduce the SetFp function in StackHandler

The FP setting is different for X32 than the other platforms as
kFPOnStackSize is double the kPointerSize and we have to clear the
higher 32 bits to 0.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/20073004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15966 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Introduce StackOperandForReturnAddress operand for X64 to access return address in the stack

R=danno@chromium.org

Review URL: https://codereview.chromium.org/20628003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15967 ce2b1a6d-e550-0410-aec6-3dcde31c8c00