MIPS: Add NaN test to cctest/test-macro-assembler-mips.

MIPS: Add NaN test to cctest/test-macro-assembler-mips.

NaN value is different on MIPS and x86 architectures, and TEST(NaNx) 
tests checks the case where a x86 NaN value is serialized into the snapshot 
on the simulator during cross compilation.

BUG=

[Paul Lind, 2014-02-10 18:10:26]

Can you elaborate in the issue description, and in a comment line in the TestNaN
function that NaN value is different on MIPS & x86, and this checks the case
where a x86  NaN value is serialized into the snapshot on the simulator.

LGTM, other than that.

[Paul Lind, 2014-02-10 19:49:50]

Committed as r19241.