VAX/testsuite: Run target testing over all the usual optimization levels

VAX/testsuite: Run target testing over all the usual optimization levels

It makes sense to use what other targets do and run all the VAX test
cases over all the usual optimization levels, so make `vax.exp' use our
`gcc-dg-runtest' rather than the generic `dg-runtest' test driver.

This breaks `pr56875.c' however, which is optimized away at levels above
`-O0' as a result of how it has been written for calculations to make no
effect:

FAIL: gcc.target/vax/pr56875.c   -O1   scan-assembler ashq .*,\\$0xffffffffffffffff,
FAIL: gcc.target/vax/pr56875.c   -O2   scan-assembler ashq .*,\\$0xffffffffffffffff,
FAIL: gcc.target/vax/pr56875.c   -O3 -g   scan-assembler ashq .*,\\$0xffffffffffffffff,
FAIL: gcc.target/vax/pr56875.c   -Os   scan-assembler ashq .*,\\$0xffffffffffffffff,
FAIL: gcc.target/vax/pr56875.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none   scan-assembler ashq .*,\\$0xffffffffffffffff,
FAIL: gcc.target/vax/pr56875.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects   scan-assembler ashq .*,\\$0xffffffffffffffff,

Rather than keeping it at `-O0' update the test case for its code to do
make effect while retaining its sense.  Also reformat it according to
our requirements.

	gcc/testsuite/
	* gcc.target/vax/vax.exp: Use `gcc-dg-runtest' rather than
	`dg-runtest'.
	* gcc.target/vax/pr56875.c (dg-options): Make empty.
	(a): Rewrite for calculations to make effect.  Reformat.