powerpc/64: Always build with 128-bit long double

The amdgpu driver builds some of its code with hard-float enabled,
whereas the rest of the kernel is built with soft-float.

When building with 64-bit long double, if soft-float and hard-float
objects are linked together, the build fails due to incompatible ABI
tags.

In the past there have been build errors in the amdgpu driver caused by
this, some of those were due to bad intermingling of soft & hard-float
code, but those issues have now all been fixed since commit 58ddbecb14c7
("drm/amd/display: move remaining FPU code to dml folder").

However it's still possible for soft & hard-float objects to end up
linked together, if the amdgpu driver is built-in to the kernel along
with the test_emulate_step.c code, which uses soft-float. That happens
in an allyesconfig build.

Currently those build errors are avoided because the amdgpu driver is
gated on 128-bit long double being enabled. But that's not a detail the
amdgpu driver should need to be aware of, and if another driver starts
using hard-float the same problem would occur.

All versions of the 64-bit ABI specify that long-double is 128-bits.
However some compilers, notably the kernel.org ones, are built to use
64-bit long double by default.

Apart from this issue of soft vs hard-float, the kernel doesn't care
what size long double is. In particular the kernel using 128-bit long
double doesn't impact userspace's ability to use 64-bit long double, as
musl does.

So always build the 64-bit kernel with 128-bit long double. That should
avoid any build errors due to the incompatible ABI tags. Excluding the
code that uses soft/hard-float, the vmlinux is identical with/without
the flag.

It does mean any code which is incorrectly intermingling soft &
hard-float code will build without error, so those bugs will need to be
caught by testing rather than at build time.

For more background see:
  - commit d11219ad53dc ("amdgpu: disable powerpc support for the newer display engine")
  - commit c653c591789b ("drm/amdgpu: Re-enable DCN for 64-bit powerpc")
  - https://lore.kernel.org/r/dab9cbd8-2626-4b99-8098-31fe76397d2d@app.fastmail.com

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://msgid.link/20230404102847.3303623-1-mpe@ellerman.id.au

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