For x86_64, gcc 7.2 under Amazon Linux AMI sets its path to x86_64-amazon-linux.

gcc 7.2 under Amazon Linux AMI sets its paths to x86_64-amazon-linux. Adding 
this triple to the list of search, plus a test case to cover this.

The patch fixes the following bug reported in bugzilla:

https://bugs.llvm.org/show_bug.cgi?id=35992

Reviewers: echristo

Differential Revision: https://reviews.llvm.org/D46230

llvm-svn: 337811