Upstream clang3.6 still doesn't allow change of calling convension of
__builtin_isnan from the default to "softfp" (via __attribute__((pcs("aapcs"))))
as required by bionic libm.so for 32-bit ARM. Let's hope clang3.7
fix that. See upstream bug http://llvm.org/bugs/show_bug.cgi?id=20958
Change-Id: Ibdcc1783ffa7cf2f36e733983bd6d912ad38df70