ARM64: Fix stack manipulation.

Builtins::Generate_StringConstructCode was claiming stack space instead of
giving it back.

BUG=chromium:425585
LOG=Y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/672623003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24815 ce2b1a6d-e550-0410-aec6-3dcde31c8c00