Add support for reducing the number of LLVM backends built.

There are three options:

BE_FREEBSD: Backends for FreeBSD architectures
BE_NATIVE: Backend(s) the native architecture
BE_STANDARD: All non-experimental backends (as configured by LLVM)

BE_STANDARD is the default and leaves the plist unchanged.

This feature is experimental and will be migrated to other devel/llvm ports
after we gain some experince with it.

Requested by: many
Pointer in the right direction: jhibbits
Sponsored by: DARPA, AFRL