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.
CTest: Use -A option to pass generator platform selection

This is less verbose than defining CMAKE_GENERATOR_PLATFORM.
ExternalProject: Use -A option to pass generator platform

This is less verbose than defining CMAKE_GENERATOR_PLATFORM.
Tests: Use -A option to pass generator platform selection

This is less verbose than defining CMAKE_GENERATOR_PLATFORM.
Tests: Test setting a generator platform in a toolchain file

Teach the RunCMake.GeneratorPlatform test to cover setting
CMAKE_GENERATOR_PLATFORM in a file loaded by CMAKE_TOOLCHAIN_FILE.
Merge topic 'vs-generator-platform' into next

c9cad78f Tests: Test setting a generator platform in a toolchain file
47f32e1b Tests: Use -A option to pass generator platform selection
c39e89db ExternalProject: Use -A option to pass generator platform
7eccc387 CTest: Use -A option to pass generator platform selection
5a6c9ab4 cmake: Add -A option to specify a generator platform