testsuite: Tweak sse2-p{add,sub}[bdw]-2.c tests for -march=cascadelake

Update the recently added gcc.target/i386/sse2-p{add,sub}[bdw]-2.c
tests for -march=cascadelake.  Committed as obvious.

2026-05-29  Roger Sayle  <roger@nextmovesoftware.com>

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-paddb-2.c: Support -march=cascadelake.
	* gcc.target/i386/sse2-paddd-2.c: Likewise.
	* gcc.target/i386/sse2-paddw-2.c: Likewise.
	* gcc.target/i386/sse2-psubb-2.c: Likewise.
	* gcc.target/i386/sse2-psubd-2.c: Likewise.
	* gcc.target/i386/sse2-psubw-2.c: Likewise.