Try to shrink kernels to smaller than ~6MB due to ARC BIOS restriction.

Currently NetBSD/arc kernels are loaded at 0x80200000
(on some machines ARC BIOS uses regions before it) and
ARC BIOS also uses around 0x807f0000, so bootloader fails
on loading >6MB kernels.

Note GXemul doesn't have this restriction because it loads
a kernel directly via a command line argument.

Tested by installing using sysinst on RAMDISK kernel on netbsd-9.

Should be pulled up to netbsd-9.

(tsutsui)