Open64 compiler (i.e. including Openf90) does use amdlibm for optimized core math functions. Open64 relies on amdlibm if you use higher optimization levels like -Ofast (starting Open64 4.2.5 even -O3 by default uses amdlibm)
Open64 does optimally pick scalar or vector amdLibm functions, you don't have to do anything specific.
Hope this clarifies.
Besides do note
- As you have noted, amdlibM is no longer shipped as part of ACML but is provided as an independent download.
- You can raise Open64 related queries under the category x86 Open64 Compiler Suite (http://forums.amd.com/devforum/categories.cfm?catid=373&entercat=y)