package/mono: fix external libatomic_ops linkage

Added patch fixes linkage of libgc with external libtomic-ops.
Mono uses bundeled bdwgc which doesn't link external libatomic-ops.
Patch is a fix cherry-picked from bdwgc upstream.

Problem found on ARMv5 processors, on newer ARM processors
it uses header based functions and doesn't need built library.

Error:
/home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: ../../external/bdwgc/.libs/libgc.a(gc.o): in function `GC_steal_mark_stack':
gc.c:(.text+0x2020): undefined reference to `AO_store_full_emulation'

Fixes:
http://autobuild.buildroot.net/results/ebc54e5dea63aca21a4072d294fdede41de559c7
http://autobuild.buildroot.net/results/6d10a4bd43fbc9c1d3fa26d5eef394c8023cb85f

Signed-off-by: Illia Bitkov <illia.bitkov@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/mono/0001-Fix-linkage-with-a-system-libatomic_ops-shared-library.patch

@@ -0,0 +1,52 @@
+From 02a44ee1df8176c72e75fd706d1a8f063d3196d5 Mon Sep 17 00:00:00 2001
+From: Fabrice Fontaine <fontaine.fabrice@gmail.com>
+Date: Mon, 5 Nov 2018 22:50:40 +0100
+Subject: Fix linkage with a system libatomic_ops shared library
+
+Issue #247 (bdwgc).
+
+When bdwgc is linked with the external libatomic_ops, bdw-gc.pc must
+contain the needed dynamic libraries (such as -latomic_ops) otherwise
+build of applications could fail on the link stage on some hosts:
+* libgc.so: undefined reference to 'AO_fetch_compare_and_swap_emulation'
+* libgc.so: undefined reference to 'AO_store_full_emulation'
+
+So, this commit sets ATOMIC_OPS_LIBS to "-latomic_ops" when a system
+atomic_ops library is used and uses ATOMIC_OPS_LIBS in bdw-gc.pc.in.
+
+* bdw-gc.pc.in (Libs): Add @ATOMIC_OPS_LIBS@.
+* configure.ac [$with_libatomic_ops!=no && $with_libatomic_ops!=none
+&& $THREADS!=none] (ATOMIC_OPS_LIBS): Set to -latomic_ops; do AC_SUBST.
+
+Fix is cherry-picked from bdwgc upstream
+
+Link to original commit https://github.com/ivmai/bdwgc/commit/02a44ee1df8176c72e75fd706d1a8f063d3196d5?branch=02a44ee1df8176c72e75fd706d1a8f063d3196d5&diff=unified  
+
+Signed-off-by: Illia Bitkov <illia.bitkov@mind.be>
+
+diff --git a/external/bdwgc/bdw-gc.pc.in b/external/bdwgc/bdw-gc.pc.in
+index ef4c2341..a32e7598 100644
+--- a/external/bdwgc/bdw-gc.pc.in
++++ b/external/bdwgc/bdw-gc.pc.in
+@@ -6,5 +6,5 @@ includedir=@includedir@
+ Name: Boehm-Demers-Weiser Conservative Garbage Collector
+ Description: A garbage collector for C and C++
+ Version: @PACKAGE_VERSION@
+-Libs: -L${libdir} -lgc
++Libs: -L${libdir} @ATOMIC_OPS_LIBS@ -lgc
+ Cflags: -I${includedir}
+diff --git a/external/bdwgc/configure.ac b/external/bdwgc/configure.ac
+index 21abe8fa..9ffe81de 100644
+--- a/external/bdwgc/configure.ac
++++ b/external/bdwgc/configure.ac
+@@ -1081,7 +1081,9 @@ AS_IF([test x"$with_libatomic_ops" = xno \
+ AC_MSG_CHECKING([which libatomic_ops to use])
+ AS_IF([test x"$with_libatomic_ops" != xno],
+   [ AS_IF([test x"$with_libatomic_ops" != xnone -a x"$THREADS" != xnone],
+-          [ AC_MSG_RESULT([external]) ],
++          [ AC_MSG_RESULT([external])
++            ATOMIC_OPS_LIBS="-latomic_ops"
++            AC_SUBST([ATOMIC_OPS_LIBS]) ],
+           [ AC_MSG_RESULT([none])
+             AS_IF([test x"$THREADS" != xnone],
+                   [ AC_DEFINE([GC_BUILTIN_ATOMIC], [1],