Configure: engage x25519 assembly support.

Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5408)

Configure

@@ -1341,6 +1341,9 @@ unless ($disabled{asm}) {
     if ($target{ec_asm_src} =~ /ecp_nistz256/) {
 	push @{$config{defines}}, "ECP_NISTZ256_ASM";
     }
+    if ($target{ec_asm_src} =~ /x25519/) {
+	push @{$config{defines}}, "X25519_ASM";
+    }
     if ($target{padlock_asm_src} ne $table{DEFAULTS}->{padlock_asm_src}) {
 	push @{$config{defines}}, "PADLOCK_ASM";
     }