cad/gmsh: fix build on powerpc64

In file included from /wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/src/common/Options.cpp:38:
In file included from /wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/src/post/adaptiveData.h:20:
In file included from /wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/src/numeric/fullMatrix.h:21:
In file included from /wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/contrib/eigen/Eigen/Dense:1:
In file included from /wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/contrib/eigen/Eigen/Core:210:
/wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/contrib/eigen/Eigen/src/Core/arch/AltiVec/PacketMath.h:1345:32: error: use of undeclared identifier 'vec_sqrt'; did you mean 'vec_rsqrt'?

BF16_TO_F32_UNARY_OP_WRAPPER(vec_sqrt, a);
                             ^~~~~~~~
                             vec_rsqrt

/wrkdirs/usr/ports/cad/gmsh/work/gmsh-4.11.0-source/contrib/eigen/Eigen/src/Core/arch/AltiVec/PacketMath.h:1302:22: note: expanded from macro 'BF16_TO_F32_UNARY_OP_WRAPPER'

Packet4f op_even = OP(a_even);\
                   ^

/usr/lib/clang/13.0.0/include/altivec.h:8472:34: note: 'vec_rsqrt' declared here
static vector float ATTRS_o_ai vec_rsqrt(vector float a) {

^