target-alpha: Ignore the unused fp_status exceptions

Mask out the exceptions that have no correspondance on
Alpha so that zero/non-zero tests work as expected.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Raise EXC_M_INV properly for fp inputs

Ignore DNZ if software completion isn't used.  Raise INV for
denormals in system mode so the OS completion handler sees them.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Suppress underflow from CVTTQ if DNZ

I.e. respect flush_inputs_to_zero.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Raise IOV from CVTQL

Even if an exception isn't taken, the status flags need updating
and the result should be written to the destination.  Move the body
of cvtql out of line, since we now always need a call.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Rename fcvtql

... to match the insn, which has no leading "f".

Signed-off-by: Richard Henderson <rth@twiddle.net>