amdgpu: Prevent build errors regarding soft/hard-float FP ABI tags

On PowerPC, the compiler will tag object files with whether they
use hard or soft float FP ABI and whether they use 64 or 128-bit
long double ABI. On systems with 64-bit long double ABI, a tag
will get emitted whenever a double is used, as on those systems
a long double is the same as a double. This will prevent linkage
as other files are being compiled with hard-float.

On ppc64, this code will never actually get used for the time
being, as the only currently existing hardware using it are the
Renoir APUs. Therefore, until this is testable and can be fixed
properly, at least make sure the build will not fail.

Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>