MSP430: Skip gcc.dg/pr55940.c in the small memory model

In the MSP430 small memory model, there is a 16-bit address space and
pointer arithmetic wraps around the address space, so any calculated
address is always within this range.

In this test, pointer arithmetic wraps when 0x1000 is added to the
address of a variable, causing the resulting address to be unexpectedly
less than 0x2000, which breaks the test.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr55940.c: Skip for msp430 unless -mlarge is specified.

gcc/testsuite/gcc.dg/pr55940.c

@@ -1,5 +1,6 @@
 /* PR target/55940 */
 /* { dg-do run } */
+/* { dg-skip-if "pointer arithmetic can wrap" { msp430-*-* } { "*" } { "-mlarge" } } */
 /* { dg-options "-Os" } */
 /* { dg-additional-options "-mpreferred-stack-boundary=2" { target { { i?86-*-* x86_64-*-* } && ia32 } } } */