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/fix-linux-header-use-in-libgcc.patch

@@ -1,44 +0,0 @@
-From 9cd4503714c4e596818c4ea8e15905c249b7a319 Mon Sep 17 00:00:00 2001
-From: nsz <nsz@138bc75d-0d04-0410-961f-82ee72b054a4>
-Date: Thu, 25 Aug 2016 14:08:27 +0000
-Subject: [PATCH] [mips] Fix linux header use in libgcc
-
-libgcc/
-	* config/mips/linux-unwind.h: Use sys/syscall.h.
-
-
-
-git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@239759 138bc75d-0d04-0410-961f-82ee72b054a4
----
- libgcc/ChangeLog                  | 4 ++++
- libgcc/config/mips/linux-unwind.h | 2 +-
- 2 files changed, 5 insertions(+), 1 deletion(-)
-
-diff --git a/libgcc/ChangeLog b/libgcc/ChangeLog
-index b78c4b2..99bd2f1 100644
---- a/libgcc/ChangeLog
-+++ b/libgcc/ChangeLog
-@@ -1,3 +1,7 @@
-+2016-08-25  Szabolcs Nagy  <szabolcs.nagy@arm.com>
-+
-+	* config/mips/linux-unwind.h: Use sys/syscall.h.
-+
- 2016-08-16  Joseph Myers  <joseph@codesourcery.com>
- 
- 	PR libgcc/77265
-diff --git a/libgcc/config/mips/linux-unwind.h b/libgcc/config/mips/linux-unwind.h
-index bf12de5..4035c121 100644
---- a/libgcc/config/mips/linux-unwind.h
-+++ b/libgcc/config/mips/linux-unwind.h
-@@ -27,7 +27,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
-    state data appropriately.  See unwind-dw2.c for the structs.  */
- 
- #include <signal.h>
--#include <asm/unistd.h>
-+#include <sys/syscall.h>
- 
- /* The third parameter to the signal handler points to something with
-  * this structure defined in asm/ucontext.h, but the name clashes with
--- 
-2.9.3
-