powerpc/64: Fix bug in setting floating-point exception mode

When loading up the FPU, we were using a 'ld' (load doubleword)
instruction to get the FP exception mode from the thread_struct,
but it's only an int field.  This changes the ld to lwz (load
word and zero-extend).

Signed-off-by: Paul Mackerras <paulus@samba.org>