Fix compilation on FreeBSD

[dnebdal]

In case you want them, these are the changes I had to do to make this compile and run on FreeBSD.

This fixes three separate issues:

1. It uses clang as CC, since that's the default compiler on FreeBSD i386 and amd64.
   I used the same prefix/suffix logic as for GCC, except that it has to handle how "clang" is typically in /usr/bin, but the suffixed versions (installed from packages) are typically in /usr/local/bin.
2. Handle uname -m returning "amd64" instead of "x86_64"
3. Detect freebsd as a nix-style OS in hisat2.

2 and 3 alone should be enough assuming you install gcc first - but the code compiles fine with clang, so that seems unnecessary. In the long run, I guess a cleaner solution would be to respect CC/CPP/CXX if those are set in the environment, and otherwise default to gcc/clang depending on either the OS or just which one is available.