kernel: Skip bss clear on native_posix

There's no point to doing this when the host OS clears all memory at
mapping time.  And as it turns out, the __bss_end symbol it was
relying on actually comes from the host toolchain's linker, not our
own linker scripts (making it semi-dangerous to rely on).  And it's
not present in clang/lld output anyway.

Signed-off-by: Andy Ross <andyross@google.com>