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
Jiading Gai