packages: rename elftoolchain to toolchain, add more things

Rename the existing "elftoolchain" package to "toolchain", and move
everything which is gated by MK_TOOLCHAIN (e.g. lex, yacc, ...) to the
toolchain package.

This means we have one package called "toolchain" which contains all the
development-related utilities which are not compilers or already part of
some other package (e.g., llvm).

Reviewed by: des, emaste
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50286