Add __amdflang__ define to identify compiler as AMD Flang

[mjklemm]

The current compiler version identifies itself as a Flang compiler:

#define __flang__ 1
#define __flang_major__ 20
#define __flang_minor__ 0
#define __flang_patchlevel__ 0

This PR adds the following define to the list:

#define __amdflang__ 1