arch/tile: avoid ISO namespace pollution with <asm/sigcontext.h>

<asm/sigcontext.h> is used by glibc's <bits/sigcontext.h> from <signal.h>,
which means that it can't clutter the namespace with random symbols
or #defines.  However, we use <arch/abi.h> to get a suitable type to
hold a machine register.

This change makes <arch/abi.h> safe to use in this kind of context
if __need_int_reg_t is defined prior to including the file; in that
case, it only defines a few symbols that are safe in the ISO namespace
(prefixed with double underscores).  <asm/sigcontext.h> then uses
the __uint_reg_t type instead of the normal uint_reg_t.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>