VAX/testsuite: Fix compilation options for `bswapdi-1.c'

Correct a regression in `vax-netbsdelf' gcc testing:

.../gcc/testsuite/gcc.target/vax/bswapdi-1.c: In function '__bswapdi2':
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:5:19: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:6:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:7:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:8:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:9:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:10:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:11:14: error: use of C99 long long integer constant [-Wlong-long]
.../gcc/testsuite/gcc.target/vax/bswapdi-1.c:12:14: error: use of C99 long long integer constant [-Wlong-long]
compiler exited with status 1
FAIL: gcc.target/vax/bswapdi-1.c (test for excess errors)

which is due to the defaults from `vax.exp':

# If a testcase doesn't have special options, use these.
global DEFAULT_CFLAGS
if ![info exists DEFAULT_CFLAGS] then {
    set DEFAULT_CFLAGS " -ansi -pedantic-errors"
}

Use an empty override then as the options used do not matter for the
objective of this test case.

	gcc/testsuite/
	* gcc.target/vax/bswapdi-1.c (dg-options): New setting.

gcc/testsuite/gcc.target/vax/bswapdi-1.c

@@ -1,3 +1,5 @@
+/* { dg-options "" } */
+
 typedef int DItype __attribute__ ((mode (DI)));
 DItype
 __bswapdi2 (DItype u)