Do not force SSE2 instructions on i386 builds

To have consistent rendering between x86 and x64 builds QtWebKit enables
SSE2 math on x86. This however raises the minimum requirements of the x86
binaries and should be disabled in Qt builds.

Change-Id: If57ff0a3c9cd0a91a508ba02fda8b596381262a8
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>