Add definition pg_attribute_aligned() for MSVC

Add definition pg_attribute_aligned() for MSVC

Visual Studio 2015+ has support for a macro to control the alignement of
structures as of __declspec(align(#)), and this commit adds a definition
of pg_attribute_aligned() based on that.  It happens that this was
already used in the implementation of atomics for MSVC.  Note that there
is still no definition fo pg_attribute_packed(), so this does not impact
itemptr.h.

Author: James Coleman
Discussion: https://postgr.es/m/CAAaqYe-HbtZvR3msoMtk+hYW2S0e0OapzMW8icSMYTMA+mN8Aw@mail.gmail.com