Merge pull request #4248 from Unity-Technologies/boehm-gc-alloc-fixed

Implement mono_gc_alloc_fixed/mono_gc_free_fixed on Boehm to match SGen

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

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

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

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

diff --cc mono/mini/debugger-agent.c
Simple merge