testsuite: Fix up builtin-prefetch-1.c tests

testsuite: Fix up builtin-prefetch-1.c tests

How can you use "read-shared" as an identifier?  It's not allowed by all
C standard versions.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/builtin-prefetch-1.c (rws): Use
	"read_shared" instead of "read-shared" as the identifier for
	enum value.
	* gcc.dg/builtin-prefetch-1.c (rws): Likewise.