Hi Andrew, Daniel,

This patch applies to the case where you have a mips64 target
(eg. mipsisa64-elf), but you're compiling in 32 bit mode (gcc -mips32).

In this case mips_regsize (gdbarch) is 64, but mips_saved_regsize (tdep)
is 32 (ie. only 32 bits are used to save a register on the stack).

trad_frame_prev_register can't handle that case (AFAICT), because
it relies on what the regcache says about the regsize.

So I've added come code to mips_mdebug_frame_prev_register
to handle it.  This fixes at least 500 testsuite failures,
and in fact makes the results for -mips32 almost identical
with the results for -mips64 (they were woefully different
before).