Partial revert of r21901 (2nd attempt)

Only disable runtime check for sse2 if __SSE2__ is not defined. This
is required for the x87 port

BUG=none
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/331803006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21938 ce2b1a6d-e550-0410-aec6-3dcde31c8c00