Support for Amazon Linux

[Rafael Espíndola]

> Unfortunately, I don't think that these are currently testable because the sysroot is not respected by DetectDistro:
>
>   if (!llvm::MemoryBuffer::getFile("/etc/redhat-release", File)) {
>     StringRef Data = File.get()->getBuffer();
>     if (Data.startswith("Fedora release"))
>       return Fedora;
>     else if (Data.startswith("Red Hat Enterprise Linux") &&
>              Data.find("release 6") != StringRef::npos)
>       return RHEL6;
> ...
>
> We'd need to change that first, no?

The patch is just updating the list of triple aliases. That is used by
the gcc installation detection logic, which can be tested.

Things that cannot be tested currently is behaviour like "pass this
linker option if running on fedora".

Cheers,
Rafael