Use C preprocessor comments in assembly headers.

We generate .S files for assembly, which means they run through the C
preprocessor first. In gas targets where # is the comment marker, there
is a conflict with cpp directives.

The comments actually rely on #This and #source not being directives. If
I begin a line with "if", the build fails. Since the C preprocessor is
responsible for removing C preprocessor comments, we should be able to
safely use // everywhere with less ambiguity.

(In fact, we were already relying on this for 32-bit ARM. The 32-bit ARM
gas line comment marker is @. 64-bit ARM uses //, and x86/x86_64/ppc64
use #.)

This reportedly causes issues for goma. See
https://bugs.chromium.org/p/boringssl/issues/detail?id=448#c3

Bug: 448
Change-Id: Ib58f3152691c1dbcccfc045f21f486b56824283d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/49965
Reviewed-by: Adam Langley <agl@google.com>

crypto/perlasm/ppc-xlate.pl

@@ -256,8 +256,8 @@ my $darn = sub {
 };
 
 print <<___;
-# This file is generated from a similarly-named Perl script in the BoringSSL
-# source tree. Do not edit by hand.
+// This file is generated from a similarly-named Perl script in the BoringSSL
+// source tree. Do not edit by hand.
 
 #if defined(__has_feature)
 #if __has_feature(memory_sanitizer) && !defined(OPENSSL_NO_ASM)

crypto/perlasm/x86_64-xlate.pl

@@ -1136,7 +1136,7 @@ my $endbranch = sub {
 ########################################################################
 
 {
-  my $comment = "#";
+  my $comment = "//";
   $comment = ";" if ($masm || $nasm);
   print <<___;
 $comment This file is generated from a similarly-named Perl script in the BoringSSL

crypto/perlasm/x86asm.pl

@@ -275,7 +275,7 @@ sub ::asciz
 
 sub ::asm_finish
 {   &file_end();
-    my $comment = "#";
+    my $comment = "//";
     $comment = ";" if ($win32);
     print <<___;
 $comment This file is generated from a similarly-named Perl script in the BoringSSL