sys-devel/llvm: Clean Python, enable libomp and BPF backend in 3.7

sys-devel/llvm: Clean Python, enable libomp and BPF backend in 3.7

Backport some of the changes from -9999 as 3.7.0-r4. Remove multi-Python
support which was working only partially because of lldb, also fixing
byte-compilation of Python modules, #567550. Default to libomp instead
of semi-broken libgomp support, #562990. Enable BPF backend by default,
GitHub PR #450.

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=567550
Fixes: https://bugs.gentoo.org/show_bug.cgi?id=562990
Fixes: https://github.com/gentoo/gentoo/pull/450