ARM: fix loading of global object in LWrapReceiver.

Since r16993 the cp register is handled by registers allocator,
and we cannot assume that the cp always contains the context.

BUG=318420
LOG=Y
TEST=test/mjsunit/regress/regress-318420.js
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/121703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18421 ce2b1a6d-e550-0410-aec6-3dcde31c8c00