Detect MIPS64 architecture

Adds WTF platform support for the MIPS64 architecture.

Patch is based on http://patches.openembedded.org/patch/51625/

Task-number: QTBUG-35394
Change-Id: I270d2d89cc1d93f06f5b40a6d1042ad5fbac2fe7
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>