Lazily save double registers for HCallRuntime instructions within Hydrogen code stubs.

Right now we eagerly save all allocatable double registers upon
entry to every Hydrogen code stub that uses HCallRuntime, and
restore them when we return. Since the HCallRuntime is on the
fallback path for code stubs, this is both a waste of time and
stack space in almost every case.

This patch adds a flag to the HCallRuntime, which controls whether
the instruction saves the double register itself (using the save
doubles flag for the CEntryStub), or whether its up the surrounding
code to handle the clobbering of double registers.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/23530066

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17044 ce2b1a6d-e550-0410-aec6-3dcde31c8c00