Improve the coverage of debug symbols for MK_DEBUG_FILES.

• Include debug symbols in static libraries. This permits binaries to include debug symbols for functions obtained from static libraries.
• Permit the C/C++ compiler flags added for MK_DEBUG_FILES to be overridden by setting DEBUG_FILES_CFLAGS. Use this to limit the debug information for llvm libraries and binaries.

Reviewed by: emaste
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D12025