Enable MK_LLVM_BINUTILS if MK_ASAN is requested

ASan will not be able to provide backtraces with symbol names with
elftoolchain's addr2line. To fix this turn MK_LLVM_BINUTILS on by
default when ASan instrumentation is requested.

Reviewed By: emaste, markj
Differential Revision: https://reviews.freebsd.org/D31061