do not require binutils port when using lld as ld
r279908 added logic to Makefile.inc1 to automatically set
CROSS_BINUTILS_PREFIX for architectures not supported by the in-tree
binutils: arm64 when first introduced, and later riscv64 as well.
LLVM's LLD linker is now included in the base system, and is enabled by
default for arm64 and capable of linking world and kernel. Thus, avoid
automatically setting CROSS_BINUTILS_PREFIX and requiring the binutils
port if WITH_LLD_IS_LD is true.
Reviewed by: kan
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10310