src.conf.5: Expand WITH_LLVM_BINUTILS description

List the specific tools that are controlled by WITH_LLVM_BINUTILS, and
mention the tools that are always or never taken from LLVM. Tools come
from one of three sources (LLVM, ELF Tool Chain, bespoke base system)
and it is useful to be explicit.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39428