NaCl ARM: fix assembler float abi flags

Summary:
tools::arm::getARMFloatABI() was falling back to guessing soft-float because
it wasn't seeing the GNUEABIHF environment from ComputeEffectivClangTriple
when it was called from gnutools::Assemble::ConstructJob.

Fix by using the effective clang triple in gnutools::Assemble, which now
matches the -triple flag used by cc1 and ClangAs jobs.

Reviewers: jvoung

Subscribers: rengolin, jfb, aemerson, cfe-commits

Differential Revision: http://reviews.llvm.org/D8902

llvm-svn: 234661