llvm-config: fix --libs on Linux

Summary:
llvm-config --libs does not produce correct output since commit r260263
(llvm-config: Add preliminary Windows support) changed naming format of
the libraries. This patch updates llvm-config to recognize new naming
format and output correct linker flags.

Ref: https://llvm.org/bugs/show_bug.cgi?id=26581

Patch by Vedran Miletić

Reviewers: ehsan, rnk, pxli168

Subscribers: pxli168

Differential Revision: http://reviews.llvm.org/D17300

llvm-svn: 263497