llvm: Don't install /usr/bin/{gcov,objdump} twice

Commit c4f08d46c7f7 moved the symlinks for the LLVM binutils from LLVM
itself to the toolchain (usr.bin/clang/toolchain), but did not remove
the links for /usr/bin/gcov and /usr/bin/objdump from the llvm version,
meaning we installed them twice, once in the clang package and once in
the toolchain package.

Remove the links from the llvm version and move the MLINKs to toolchain,
which is where the other MLINKs dwell.

While here, fix toolchain to use the correct build option for llvm-cov,
MK_LLVM_COV.

Fixes: c4f08d46c7f7 ("llvm-*: Move all LLVM_BINUTILS symlinks to toolchain package")
Reported by: jrm
Reviewed by: jrm, dim, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56788