ec/asm/ecp_nistz256-avx2.pl: harmonize clang version detection.

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6499)

crypto/ec/asm/ecp_nistz256-avx2.pl

@@ -47,7 +47,7 @@ if (!$addx && $win64 && ($flavour =~ /masm/ || $ENV{ASM} =~ /ml64/) &&
 	$addx = ($1>=12);
 }
 
-if (!$addx && `$ENV{CC} -v 2>&1` =~ /(^clang version|based on LLVM) ([3-9])\.([0-9]+)/) {
+if (!$addx && `$ENV{CC} -v 2>&1` =~ /((?:^clang|LLVM) version|based on LLVM) ([3-9])\.([0-9]+)/) {
 	my $ver = $2 + $3/100.0;	# 3.1->3.01, 3.10->3.10
 	$avx = ($ver>=3.0) + ($ver>=3.01);
 	$addx = ($ver>=3.03);