Added fix for Bug 20869
[tests] gc-descriptors: Only copy file in case of VPATH build

Instead of ignoring the cp error when copying over the same file in non-VPATH builds.
Merge pull request #1127 from oleg-st/master

Added fix for Bug 20869
Merge pull request #1143 from akoeplinger/fix-gc-descriptor-error

[tests] gc-descriptors: Only copy file in case of VPATH build
[mini] Simplify the way the JIT emits atomic opcodes.

* MONO_ARCH_HAVE_ATOMIC_* macros have been removed.
* Backends now report whether they support atomic opcodes via
  the mono_arch_opcode_supported () function.
* The default implementation of mono_arch_opcode_supported ()
  has been removed as almost all backends need to define one
  anyway.
* All old atomic_add_* opcodes have been removed. These were no
  longer being used but some backends still had code for them.
* atomic_add_new_* opcodes have been renamed to atomic_add_* as
  they are now the only atomic add opcodes.
* Itanium-specific atomic_add_imm_* opcodes have been moved to
  the __ia64__ section of mini-ops.h.
* Comments have been added to mini-ops.h explaining the precise
  semantics of the atomic opcodes.

The net result of this change is a simpler backend interface at
the cost of a slightly bigger binary.
[ppc] Clean up atomic opcodes.

* Support atomic_add_i4 on ppc32 in addition to ppc64.
* Make the atomic_add_* code more in line with the atomic_cas_* code.
[corlib] Re-enable TaskContinuationChainLeak test.

This is hopefully fixed after a57fb5b.
[runtime] Fix the android build.
Remove duplicate System.ServiceModel in mcs/class/Makefile.

This was causing tests to be run several times in the same profile(s).
Remove duplicate System.Web in mcs/class/Makefile.

It was listed both in net_2_0_dirs and common_dirs so
it would get tested twice in some profiles.
Revert "Remove duplicate System.Web in mcs/class/Makefile."

This reverts commit 5fbb351375a38232ff9cb5ca6602f183321e0a47.

This breaks the build because System.Runtime.Remoting needs System.Web.
mcs/class/Makefile: Merge common_dirs into net_2_0_dirs.

They're used in the same places so we may as well simplify.
mcs/class/Makefile: Remove duplicate System.Web entry.

Second take on 5fbb351375a38232ff9cb5ca6602f183321e0a47.