SEGV on display /i $pc with i386 target

[Christopher Faylor <cgf@redhat.com>]

I just noticed a SEGV whenever I do a 'display /i $pc' on cygwin.

I tried building a gdb for linux to see what was going wrong on
cygwin but it isn't much better:

(top-gdb) display /i $pc
1: x/i $(null)  0x8072f42 <main+6>:     push   $0x6

The problem comes from the fact that, while gdb understands that
$pc == $eip, it doesn't seem to know how to rename $pc to $eip
when it is outputting the register name.  You can get the same
behavior by doing something like 'display /i $ps', too (even
if that doesn't make sense it shouldn't SEGV).

The simplest way to fix this is to extend the i386_register_names array
to include builtin register names, however, maybe the right way to fix
this is to add something to builtin-reg.c.

I noticed that i386_register_names seems to have 41 elements while
the sum of NUM_REGS + NUM_PSEUDO_REGS == 40.  Is that intentional?

cgf