Change CMake to respect GPU_TARGETS variable

[lawruble13]

Summary of proposed changes:

• Change the CMake to set the GPU_TARGETS variable from AMDGPU_TARGETS if not set by the user
• Change all usages of AMDGPU_TARGETS to GPU_TARGETS

[ntrost57]

I guess we can also remove AMDGPU_TARGETS?

[jsandham]

I think we are supposed to continue supporting AMDGPU_TARGETS but just issue a deprecation warning saying to use GPU_TARGETS. I think we want to add something like what rocblas did, i.e. delete the above line and add this instead:

if( AMDGPU_TARGETS AND NOT GPU_TARGETS )
message( DEPRECATION "AMDGPU_TARGETS use is deprecated. Use GPU_TARGETS." )
set( GFX_LIST ${AMDGPU_TARGETS} )
elseif( GPU_TARGETS )
set( GFX_LIST ${GPU_TARGETS} )
else ()
set( GFX_LIST ${DEFAULT_AMDGPU_TARGETS} ) # default if not set
endif()

set(GPU_TARGETS "${GFX_LIST}" CACHE STRING "GPU architectures to build for")