package/mono: fix external libatomic_ops linkage

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>