Update diff to be in standard format (had been created with "diff.mnemonicprefix=true").

In D46876#1068975, @emaste wrote:

phabricator tagged it with "does not need to be reviewed"

Phabricator recognizes it as a generated file and so doesn't need to be reviewed. The note is intended more for the assigned reviewers - the change can be included, Phab just notes that reviewers don't need to look at it.

Address review comments.

In D46876#1068925, @des wrote:

You also need to remove tools/build/options/WITH_GH_BC, move the relevant section of tools/build/mk/OptionalObsoleteFiles.inc to ObsoleteFiles.inc, and regenerate share/man/man5/src.conf.5.

In D46876#1068928, @des wrote:

In D46876#1068845, @emaste wrote:

Is there value in keeping the tests that were added to the old version?

Perhaps. More importantly, Gavin Howard's bc has its own test suite which we could adopt. Take a look at how Apple did it: https://github.com/apple-oss-distributions/bc

Update diff to include removal of dependency on MK_GH_BC from usr.bin/Makefile and from tools/build/mk/OptionalObsoleteFiles.inc.