[MIPS] Call AddBiasForPNaCl for float_rounding_mode_test
[MIPS] Call AddBiasForPNaCl for float_rounding_mode_test

Minor change to tests/math/nacl.scons to call AddBiasForPNaCl to add
architecture specific defines for the target architecture. This is
important for rounding_mode_test for MIPS arch, because
float_rounding_mode.c uses

#if defined(__MIPS__)
#endif

BUG= run_float_rounding_mode_test fails for MIPS
TEST= ./scons.py platform=mips32 run_float_rounding_mode_test

Review URL: https://codereview.chromium.org/27222007

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12256 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client