chacha/asm/chacha-x86.pl: make it compile on legacy systems.

Usage of $ymm variable is a bit misleading here, it doesn't refer
to %ymm register bank, but rather to VEX instruction encoding,
which AMD XOP code path depends on.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/chacha/asm/chacha-x86.pl

@@ -437,8 +437,10 @@ my ($ap,$bp,$cp,$dp)=map(($_&~3)+(($_-1)&3),($ai,$bi,$ci,$di));	# previous
 
 &function_begin("ChaCha20_ssse3");
 &set_label("ssse3_shortcut");
+if ($ymm) {
 	&test		(&DWP(4,"ebp"),1<<11);		# test XOP bit
 	&jnz		(&label("xop_shortcut"));
+}
 
 	&mov		($out,&wparam(0));
 	&mov		($inp,&wparam(1));
@@ -770,7 +772,7 @@ sub SSSE3ROUND {	# critical path is 20 "SIMD ticks" per round
 }
 &asciz	("ChaCha20 for x86, CRYPTOGAMS by <appro\@openssl.org>");
 
-if ($xmm) {
+if ($ymm) {
 my ($xa,$xa_,$xb,$xb_,$xc,$xc_,$xd,$xd_)=map("xmm$_",(0..7));
 my ($out,$inp,$len)=("edi","esi","ecx");