main/gcc6: delete in favor of Alpine's gcc6

Alpine Linux ships their own GCC6 package now (side by side with GCC8):
https://git.alpinelinux.org/cgit/aports/commit/?id=f76673fb030820b2053cad7e5b81ece31079d4cc

main/gcc6/204-linux_libc_has_function.patch

@@ -1,25 +0,0 @@
-From edec78452d693fb524daa9a6efd45c850b27b25c Mon Sep 17 00:00:00 2001
-From: Szabolcs Nagy <nsz@port70.net>
-Date: Fri, 6 Nov 2015 23:59:20 +0000
-Subject: [PATCH 4/6] linux_libc_has_function
-
----
- gcc/config/linux.c | 2 +-
- 1 file changed, 1 insertion(+), 1 deletion(-)
-
-diff --git a/gcc/config/linux.c b/gcc/config/linux.c
-index 250296b..16c3768 100644
---- a/gcc/config/linux.c
-+++ b/gcc/config/linux.c
-@@ -26,7 +26,7 @@ along with GCC; see the file COPYING3.  If not see
- bool
- linux_libc_has_function (enum function_class fn_class)
- {
--  if (OPTION_GLIBC)
-+  if (OPTION_GLIBC || OPTION_MUSL)
-     return true;
-   if (OPTION_BIONIC)
-     if (fn_class == function_c94
--- 
-2.8.1
-