Make mips_postboot_fixup work when building the kernel with clang+lld

The compiler/linker can align fake_preload anyway it would like. When
building the kernel with gcc+bfd this always happened to be a multiple of 8.
When I built the kernel with clang and linked with lld fake_preload
happened to only be aligned to 4 bytes which caused a an ADDRS trap because
the compiler will emit sd instructions to store to this buffer.

Reviewed By: jhb, imp
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D14018