stand: Compile out the extensive superblock diagnostic messages for BIOS loader

The BIOS loader operates in a very constrained environment. The messages
for the super block integrity tests take up about 12k of space. Compile
them out for the BIOS loader, while leaving it intact for all other
loaders that aren't space constrained. These aren't used in the 'super
tiny' *boot* programs, so no adjustment is needed there.

We reply on the fact that (a) i386 doesn't support 32-bit UEFI booting
and (b) LIBSA_CPUARCH is "i386" when building on both i386 and when
we're building the 32-bit libsa32 library.

This saves about 12k of space for this constrained envrionment and will
take a bit of the pressure off some machines where the loader has grown
too big for their BIOS (see comments in i386/loader/Makefile for
details).

Sponsored by: Netflix
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D36175