perlasm/x86*.pl: add endbranch instruction.

For further information see "Control-flow Enforcement Technology
Preview" by Intel.
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/perlasm/x86_64-xlate.pl

@@ -864,6 +864,10 @@ my $vprotq = sub {
     }
 };
 
+my $endbranch = sub {
+    (0xf3,0x0f,0x1e,0xfa);
+};
+
 if ($nasm) {
     print <<___;
 default	rel

crypto/perlasm/x86asm.pl

@@ -172,6 +172,11 @@ sub ::vprotd
     {	&::generic("vprotd",@_);	}
 }
 
+sub ::endbranch
+{
+    &::data_byte(0xf3,0x0f,0x1e,0xfb);
+}
+
 # label management
 $lbdecor="L";		# local label decoration, set by package
 $label="000";