Cleanup GHC ports:

• Switch to using ld instead of binutils ld or ld.bfd on amd64/i386.
• Add GMP option.
• Re-enable DYNAMIC option on ARM. It is reported that it works now.
• Fix create-bootstrap target to produce tarballs ready to be consumed by ports.
• Update bootstrap compilers to 8.4.3 on amd64, i386, aarch64.
• Get rid merge build.boot.mk.in into build.mk.in.
• Sort some Makefile variables.

Reviewed by: Mikael Urankar <mikael.urankar@gmail.com> (ARM bits), tcberner
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D18378