bsd.linker.mk: Use :C not :S for regex

Whilst ^ and $ are supported with C, those are special cases, and
general regex syntax like groups and alternations are not. Use the
correct modifier so we get a version number out that's not 0 (which is
what happens when it can't be parsed by the later code).

Fixes: c4177f5b41d4 ("bsd.linker.mk: Handle Xcode 15 linker identification")
MFC after: 1 week