libressl: disable asm on 10.6 and below

Ref: https://github.com/libressl-portable/portable/issues/121

Closes Homebrew/homebrew#43013.

Formula/libressl.rb

@@ -12,6 +12,7 @@ class Libressl < Formula
 
   head do
     url "https://github.com/libressl-portable/portable.git"
+
     depends_on "automake" => :build
     depends_on "autoconf" => :build
     depends_on "libtool" => :build
@@ -28,6 +29,9 @@ class Libressl < Formula
       --sysconfdir=#{etc}/libressl
     ]
 
+    # https://github.com/libressl-portable/portable/issues/121
+    args << "--disable-asm" if MacOS.version <= :snow_leopard
+
     system "./autogen.sh" if build.head?
     system "./configure", *args
     system "make"