[MIPS] Fixing FP registers fs0 and fs1 corruption bug.

For performance reasons, we don't want to use "callee saved" FP registers.
The fix is to just use temporary FP registers instead.

Change-Id: Ifae20c707ff7022905a080892348bfbc151e4935
Signed-off-by: Douglas Leung <douglas@mips.com>