Fix crash with unsupported architectures in Linux/Gnu target triples.

Summary: This patch adds a check and an error message to gnutools::Linker::ConstructJob in case the architecture is not supported. For most other operating systems, the error message is created in lib/Basic/Targets.cpp:AllocateTarget, but when construction the linker arguments for the gnutools linker a supported architecture is required.

Reviewers: rafael, joerg, echristo

Subscribers: mehdi_amini, joerg, dschuff, cfe-commits

Differential Revision: https://reviews.llvm.org/D27066

llvm-svn: 288327