Silence annoying and incorrect non-default linker warning with GCC

The CROSS_TOOLCHAIN GCC .mk files include -B${CROSS_BINUTILS_PREFIX}, so
GCC will select the right linker and we don't need to warn.
While here also apply 17b8b8fb5fc4acc832dabfe7ef11e3e1d399ad0f to kern.mk.

Test Plan: no more warning printed with CROSS_TOOLCHAIN=mips-gcc6
Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D29015