[testsuite] [sparc] use -mtune in alignment tuning test

If -mcpu=leon3 is present in the command line for a test run,
overriding it with -mcpu=niagara7 is not enough to override the tuning
for leon3 selected by the previous -mcpu option.

niagara7-align.c tests for niagara7 alignment tuning, so use -mtune
rather than -mcpu.


for  gcc/testsuite/ChangeLog

	* gcc.target/sparc/niagara7-align.c: Use -mtune.

gcc/testsuite/gcc.target/sparc/niagara7-align.c

 /* { dg-do compile } */
-/* { dg-options "-falign-functions -mcpu=niagara7" } */
+/* { dg-options "-falign-functions -mtune=niagara7" } */
 /* { dg-final { scan-assembler "\.align 64" } } */
 void foo(void) {}