chacha20_poly1305_x86_64.pl: Suppress Yasm non-local label warnings.

Before, attempting to build the code using Yasm as the assembler would
result in warnings like this:

    warning : no non-local label before `.chacha20_consts'

Precede the local labels with a non-local label to suppress these
warnings.

It isn't clear why these labels are defined as local labels instead of
regular labels.  Making them non-local may be a better idea.

For reference, Yasm's interpretation of local labels is described
succinctly at
https://www.tortall.net/projects/yasm/manual/html/nasm-local-label.html.

Change-Id: Ifc92de7fd7379859fe33f1137ab20b6ec282cd0b
Reviewed-on: https://boringssl-review.googlesource.com/13384
Reviewed-by: Adam Langley <agl@google.com>

crypto/cipher/asm/chacha20_poly1305_x86_64.pl

@@ -39,6 +39,9 @@ $avx = 2;
 $code.=<<___;
 .text
 .extern OPENSSL_ia32cap_P
+
+chacha20_poly1305_constants:
+
 .align 64
 .chacha20_consts:
 .byte 'e','x','p','a','n','d',' ','3','2','-','b','y','t','e',' ','k'