AIX: Add platform id variables 'AIX' and 'CMAKE_HOST_AIX'

This allows projects to use `if(AIX)` blocks instead of spelling
out `if(CMAKE_SYSTEM_NAME STREQUAL "AIX")`.  We already have similar
variables for many other platforms.

Modules/CMakeSystemSpecificInitialize.cmake

@@ -17,6 +17,7 @@ unset(MSYS)
 unset(WIN32)
 unset(BSD)
 unset(LINUX)
+unset(AIX)
 
 # The CMAKE_EFFECTIVE_SYSTEM_NAME is used to load compiler and compiler
 # wrapper configuration files. By default it equals to CMAKE_SYSTEM_NAME

Modules/Platform/AIX-Initialize.cmake

@@ -1 +1,2 @@
+set(AIX 1)
 set(UNIX 1)