Unify Solaris procfs and largefile handling

GDB currently doesn't build on 32-bit Solaris:

* On Solaris 11.4/x86:

In file included from /usr/include/sys/procfs.h:26,
                 from /vol/src/gnu/gdb/hg/master/dist/gdb/i386-sol2-nat.c:24:
/usr/include/sys/old_procfs.h:31:2: error: #error "Cannot use procfs in the large file compilation environment"
 #error "Cannot use procfs in the large file compilation environment"
  ^~~~~

* On Solaris 11.3/x86 there are several more instances of this.

The interaction between procfs and large-file support historically has
been a royal mess on Solaris:

* There are two versions of the procfs interface:

** The old ioctl-based /proc, deprecated and not used any longer in
   either gdb or binutils.

** The `new' (introduced in Solaris 2.6, 1997) structured /proc.

* There are two headers one can possibly include:

** <procfs.h> which only provides the structured /proc, definining
   _STRUCTURED_PROC=1 and then including ...

** <sys/procfs.h> which defaults to _ST...