gh-bc: don't disable LTO on powerpc64

Summary:
The LTO issue has been fixed. While -flto for some reason is commented out,
since it wasn't completely removed, it may be expected to be reenabled.

Reviewers: se
Approved by: se
MFC after: 3 days
Subscribers: imp
Differential Revision: https://reviews.freebsd.org/D38755