amdgpu: fix clang build warning

clang warns about the -mhard-float command line arguments
on architectures that do not support this:

clang: error: argument unused during compilation: '-mhard-float' [-Werror,-Wunused-command-line-argument]

Move this into the gcc-specific arguments.

Fixes: e77165bf ("drm/amd/display: Add DCN3 blocks to Makefile")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>