mips: ubldr: use truncated load address for mips32

BFD appears to silently truncate 0xffffffff80800000 when it processes the
ldscript for 32-bit mips, but LLD chokes on it as the linker script tries to
place elements above 32-bit range. It's unclear to me if silent truncation
is kosher or not and whether this patch is really what we want to do, but it
is one approach at least.

Reviewed by: imp, mizhka
Differential Revision: https://reviews.freebsd.org/D21487