cmake: Add -A option to specify a generator platform

Define the 'cmake -A' option to set CMAKE_GENERATOR_PLATFORM
without having to spell out the whole variable name.  We choose
the name '-A' for "platform" because '-P' is already taken, and
in the common use case the "platform" is actually an architecture
(e.g. x64).

Teach the RunCMake test infrastructure to use -A to pass the generator
platform.  Extend the RunCMake.GeneratorPlatform test with a case to
verify that the -A option cannot be repeated.

Help/manual/OPTIONS_BUILD.txt

@@ -51,6 +51,17 @@
 
  See native build system documentation for allowed toolset names.
 
+``-A <platform-name>``
+ Specify platform name if supported by generator.
+
+ Some CMake generators support a platform name to be given to the
+ native build system to choose a compiler or SDK.  This is supported only on
+ specific generators::
+
+   Visual Studio >= 8
+
+ See native build system documentation for allowed platform names.
+
 ``-Wno-dev``
  Suppress developer warnings.
 

Help/release/dev/vs-generator-platform.rst

@@ -4,4 +4,6 @@ vs-generator-platform
 * The Visual Studio generators for versions 8 (2005) and above
   learned to read the target platform name from a new
   :variable:`CMAKE_GENERATOR_PLATFORM` variable when it is
-  not specified as part of the generator name.
+  not specified as part of the generator name.  The platform
+  name may be specified on the :manual:`cmake(1)` command line
+  with the ``-A`` option, e.g. ``-G "Visual Studio 12 2013" -A x64``.

Help/variable/CMAKE_GENERATOR_PLATFORM.rst

@@ -5,6 +5,8 @@ Generator-specific target platform name specified by user.
 
 Some CMake generators support a target platform name to be given
 to the native build system to choose a compiler toolchain.
+If the user specifies a toolset name (e.g. via the cmake -A option)
+the value will be available in this variable.
 
 The value of this variable should never be modified by project code.
 A toolchain file specified by the :variable:`CMAKE_TOOLCHAIN_FILE`