Add ASM platform information for GNU compiler on AIX (#13390)

Among other flags this sets RPATH flags correctly so that CMake knows
how to treat CMAKE_PLATFORM_REQUIRED_RUNTIME_PATH for the ASM language.
This is the GNU compiler equivalent to commit a0bab7ae (Add ASM platform
information for XL compiler on AIX, 2011-03-02), made for XL.

Modules/Platform/AIX-GNU-ASM.cmake

+include(Platform/AIX-GNU)
+__aix_compiler_gnu(ASM)