x86: Always generate .note.gnu.property section for ELF outputs

We should always generate .note.gnu.property section in x86 assembly
codes for ELF outputs to mark Intel CET support since all input files
must be marked with Intel CET support in order for linker to mark output
with Intel CET support.

Verified with

$ CC="gcc -Wl,-z,cet-report=error" ./Configure shared linux-x86 -fcf-protection
$ make
$ make test
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11044)

crypto/perlasm/x86gas.pl

@@ -173,6 +173,26 @@ sub ::file_end
 	else		{ push (@out,"$tmp\n"); }
     }
     push(@out,$initseg) if ($initseg);
+    if ($::elf) {
+	push(@out,"
+	.section \".note.gnu.property\", \"a\"
+	.p2align 2
+	.long 1f - 0f
+	.long 4f - 1f
+	.long 5
+0:
+	.asciz \"GNU\"
+1:
+	.p2align 2
+	.long 0xc0000002
+	.long 3f - 2f
+2:
+	.long 3
+3:
+	.p2align 2
+4:
+");
+    }
 }
 
 sub ::data_byte	{   push(@out,".byte\t".join(',',@_)."\n");   }