RecordWriteStubs on ARM need to save FP registers

A previous change erroneously stopped saving fp registers for write stubs in
snapshotted code. This CL restores correct behavior, and makes sure code that
saves fp registers checks at runtime for 16 or 32 fp registers. Also fix a bug
in the arm simulator debugger.

BUG=

Review URL: https://codereview.chromium.org/14246032

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14359 ce2b1a6d-e550-0410-aec6-3dcde31c8c00