[arm+x86] Make GNU variants behave like GNU w.r.t combining sin+cos into sincos.

Summary:
canCombineSinCosLibcall() would previously combine sin+cos into sincos for
GNUX32/GNUEABI/GNUEABIHF regardless of whether UnsafeFPMath were set or not.
However, GNU would only combine them for UnsafeFPMath because sincos does not
set errno like sin and cos do. It seems likely that this is an oversight.

Reviewers: t.p.northover

Subscribers: t.p.northover, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D21431

llvm-svn: 273259