Misc fixes for cross-building amd64.

Misc fixes for cross-building amd64.

Use the proper IMAKEENV instead of CROSSENV when installing a 64 bit
kernel via installkernel.  IMAKEENV supplies the proper cross-building
path so objcopy works properly.

Remove the -mcmodel option from the compilation of genassym.c.  This
utility is compiled for the host OS, not the target.

Add MACHINE_ARCH, MACHINE, and MACHINE_PLATFORM to the module build
called from the kernel build.

Add -fno-omit-frame-pointer to kmod.mk for amd64 builds (from FreeBSD).