Use upstream suffixes for LLVM IR

In r307676, several make rules were added for LLVM IR files, both in
text and binary format. Unfortunately these use different suffixes from
what upstream uses:

• Text IR has upstream suffix ".ll", while r307676 uses ".llo"
• Binary IR has upstream suffix ".bc", while r307676 uses ".bco"

Change these to what upstream uses instead.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D8326