bsd.port.mk: also export GNU strip when USE_BINUTILS is set

GNU Binutils packages strip, and should be used in preference to the base
LLVM strip when USE_BINUTILS = yes.

Do not take over the STRIP makefile variable, which is already used in
bsd.port.mk to signal whether the install program should strip the
binaries. Instead define STRIP_CMD and use STRIP just for the environment
variables.

Reviewed by: dim arrowd
Approved by: mat (portmgr)
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D55509