SlotRef::GetValue INT32 case needs to be 64bit big endian aware

On 64bit big endian systems fetch the correct 32bits from the slot

BUG=v8:3449
LOG=N
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/403003002

Patch from Andrew Low <andrew_low@ca.ibm.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22522 ce2b1a6d-e550-0410-aec6-3dcde31c8c00