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