Add gcc6 aport

Based on the last version from Alpine, but installed to /usr/gcc6 and
with pretty much everything disabled except for the C compiler.

We can probably throw out a lot of patches, and make the APKBUILD much
smaller. But let's focus on making it possible to compile old kernels
again first. There's still a lot to do for that, see #103.

main/gcc6/fix-linux-header-use-in-libgcc.patch

@@ -0,0 +1,44 @@
+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
+