tile: avoid undefined behavior with regs[TREG_TP] etc

Recent compilers warn about accesses to arrays past the end,
which is supported for pt_regs and sigcontext with the
intended alias past the end of regs[] to sp, tp, and lr
using TREG_SP, TREG_TP, and TREG_LR.

Make the intended usage explict by providing an anonymous
union of regs[56] on the one hand, and a short __regs[53]
on the other followed by the sp, tp, and lr fields, so the
aliasing is done explicitly and is visible to the compiler.

Reviewed-by: Jeff Epler <jepler@unpythonic.net>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>