target-alpha: Tidy exception constants.

target-alpha: Tidy exception constants.

There's no need to attempt to match EXCP_* values with PALcode entry
point offsets.  Instead, compress all the values to make for more
efficient switch statements within QEMU.

We will be doing TLB fill within QEMU proper, not within the PALcode,
so all of the ITB/DTB miss, double fault, and access exceptions can
be compressed to EXCP_MMFAULT.

Compress all of the EXCP_CALL_PAL exceptions into one.
Use env->error_code to store the specific entry point.

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