bsd.compiler.mk: Fix cross-building from non-FreeBSD

bsd.compiler.mk: Fix cross-building from non-FreeBSD

On non-FreeBSD, the various MACHINE variables for the host when
bootstrapping can be missing or not match FreeBSD's naming, causing
bsd.endian.mk to be unable to infer the endianness. Work around this by
assuming it's unsupported.

Note that we can't check BOOTSTRAPPING here as Makefile.inc1 includes
bsd.compiler.mk before that is set, and so we are unable to catch errors
during buildworld itself when cross-building and bsd.endian.mk failed,
but such errors should also show up when building on FreeBSD.

Fixes:	47363e99d3d3 ("Enable compressed debug on little-endian targets")