xlc 16.1 can't interpret asm lines in Eigen TPL

@utkarsh.ayachit @ben.boeckel

Issue found while debugging #20454 (closed). The workaround for that issue is needed, or you will run into those errors first. Once that is fixed the xlc build now fails with:

In file included from /home/stam/projects/paraview/src/VTK/Filters/Statistics/vtkPCAStatistics.cxx:19:
In file included from /home/stam/projects/paraview/src/VTK/ThirdParty/eigen/vtkeigen/eigen/Dense:1:
In file included from /home/stam/projects/paraview/src/VTK/ThirdParty/eigen/vtkeigen/eigen/Core:399:
/home/stam/projects/paraview/src/VTK/ThirdParty/eigen/vtkeigen/eigen/src/Core/arch/AltiVec/PacketMath.h:941:71: error: 1540-0557 The asm operand does not match the specified constraint.
  __asm__ ("xvcmpgtdp %x0,%x2,%x1\n\txxsel %x0,%x1,%x2,%x0" : "=&wa" (ret) : "wa" (a), "wa" (b));
                                                                      ^
1 error generated.
Error while processing /home/stam/projects/paraview/src/VTK/Filters/Statistics/vtkPCAStatistics.cxx.

Eigen devs are aware of the issue: https://gitlab.com/libeigen/eigen/-/issues/1555#note_435626297

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