packages: rename elftoolchain to toolchain, add more things

[ivy]

Summary

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).

[ivy]

if this lands i will send a heads up to current@ and pkgbase@ since we don't really have a good way to handle adding new packages right now.

[emaste]

I think we may want to eventually support picking one of ELF Tool Chain / llvm-binutils / GNU binutils via installing the appropriate package, but right now this change makes sense. We've switched to installing LLVM's version of most of these tools like nm, addr2line, etc., so the elftoolchain package will be mostly empty. (We build ELFTC's elfcopy because llvm-objcopy doesn't support ELF->PE conversion used by UEFI loader bits).