Fix make delete-old for gperf and GCC/CXX options

• Optional components go in OptionalObsoleteFiles
• Move gperf removal to be based on MK_GCC only, not MK_CXX and MK_GCC

Reviewed by: imp, sbruno
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2421