testsuite: -mbig/-mlittle only is valid for powerpc-linux.

testsuite: -mbig/-mlittle only is valid for powerpc-linux.

A recent change to some powerpc tests added explicit -mbig and -mlittle
options, but those options are not valid outside of powerpc-linux.

This patch updates the testcase options to enable -mbig when valid
and to only use -mlittle for powerpc-linux.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/builtins-1.c: Limit -mbig.
	* gcc.target/powerpc/vsu/vec-cntlz-lsbb-0.c: Limit -mbig.
	* gcc.target/powerpc/vsu/vec-cntlz-lsbb-1.c: Limit -mbig.
	* gcc.target/powerpc/vsu/vec-cntlz-lsbb-2.c: Remove target selector.
	* gcc.target/powerpc/vsu/vec-cntlz-lsbb-3.c: Only powerpc*-linux.
	* gcc.target/powerpc/vsu/vec-cntlz-lsbb-4.c: Only powerpc*-linux*.
	* gcc.target/powerpc/vsu/vec-cnttz-lsbb-0.c: Limit -mbig.
	* gcc.target/powerpc/vsu/vec-cnttz-lsbb-1.c: Limit -mbig.
	* gcc.target/powerpc/vsu/vec-cnttz-lsbb-2.c: Remove target selector.
	* gcc.target/powerpc/vsu/vec-cnttz-lsbb-3.c: Only powerpc*-linux*.
	* gcc.target/powerpc/vsu/vec-cnttz-lsbb-4.c: Only powerpc*-linux*.