Fix bogus builtin-snprintf-warn-3.c failure for avr

Fix bogus builtin-snprintf-warn-3.c failure for avr

The test declares a struct with an array member that has INT_MAX/32767
elements. This causes a "type xxx is too large" error for targets like
the avr, which has 16 bit pointers.

Fix by marking the test as unsupported for targets with ptr size < 32.

2017-05-23  Senthil Kumar Selvaraj  <senthil_kumar.selvaraj@atmel.com>

	* gcc.dg/tree-ssa/builtin-snprintf-warn-3.c: Require ptr32plus.

From-SVN: r248361