rs6000: Correct several errant dg-require-effective-target

I misspelled the dg-require-effective-target attribute "vsx_hw" in
recent commits, causing the effected tests to fail.  Correct the spelling.

2021-10-11  Paul A. Clarke  <pc@us.ibm.com>

gcc/testsuite
	* gcc.target/powerpc/pr78102.c: Fix dg-require-effective-target.
	* gcc.target/powerpc/sse4_1-packusdw.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmaxsb.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmaxsd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmaxud.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmaxuw.c: Likewise.
	* gcc.target/powerpc/sse4_1-pminsb.c: Likewise.
	* gcc.target/powerpc/sse4_1-pminsd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pminud.c: Likewise.
	* gcc.target/powerpc/sse4_1-pminuw.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovsxbd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovsxbw.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovsxwd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxbd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxbq.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxbw.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxdq.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxwd.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmovzxwq.c: Likewise.
	* gcc.target/powerpc/sse4_1-pmulld.c: Likewise.
	* gcc.target/powerpc/sse4_2-pcmpgtq.c: Likewise.
	* gcc.target/powerpc/sse4_1-phminposuw.c: Use correct
	dg-require-effective-target.