RFC: adding __jbslot_t to <machine/ansi.h>

[Matt Thomas <matt%3am-software.com@localhost>]

While looking at adding MIPS N32 ABI support to NetBSD, one of the  
impediments I've stumbled across is the use of long which defining the  
various jmp_buf variants.  In N32, long is not large enough to contain  
a register value since registers are 64bits long.

To fix this, I want to add __jbslot_t to <machine/ansi.h> which will  
be typedef'ed to long except which N32 is used on MIPS.

I'll then change <setjmp.h> to use jbslot_t instead of long.

Comments?