Merge pull request #1588 from BrzVlad/feature-aot-wbarrier

AOT write barrier method

diff --cc mono/metadata/marshal.c
Simple merge

diff --cc mono/metadata/sgen-gc.c
Simple merge

diff --cc mono/metadata/sgen-gc.h
Simple merge

diff --cc mono/mini/aot-compiler.c
Simple merge

diff --cc mono/mini/method-to-ir.c
Simple merge

diff --cc mono/mini/mini-runtime.c
Simple merge