fix build on P9

[jglaser]

This PR fixes the following build error when compiling grpc on Summit / Power9:

[ 10%] Building CXX object third_party/abseil-cpp/absl/random/CMakeFiles/absl_random_internal_randen_hwaes_impl.dir/internal/randen_hwaes.cc.o
[ 10%] Building CXX object third_party/protobuf/CMakeFiles/libprotobuf.dir/__/src/google/protobuf/io/zero_copy_stream_impl.cc.o
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc: In member function 'constexpr __vector(4) __bool int {anonymous}::u64x2::operator==(const {anonymous}::u64x2&) const':
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:323:51: error: cannot convert 'bool' to '__vector(4) __bool int' in return
     return v[0] == other.v[0] && v[1] == other.v[1];
                                                   ^
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:324:3: error: body of constexpr function 'constexpr __vector(4) __bool int {anonymous}::u64x2::operator==(const {anonymous}::u64x2&) const' not a return-statement
   }
   ^
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc: At global scope:
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:389:1: error: non-constant condition for static assertion
 static_assert(round_keys[kKeys - 1] != u64x2(0, 0),
 ^~~~~~~~~~~~~
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:389:37:   in constexpr expansion of '{anonymous}::round_keys[(136 - 1)].{anonymous}::u64x2::operator!=({anonymous}::u64x2(0, 0)'
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:327:20: error: constexpr __vector(4) __bool int {anonymous}::u64x2::operator==(const {anonymous}::u64x2&) const' called in a constant expression
     return !(*this == other);
             ~~~~~~~^~~~~~~~~
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc: In function '__vector(4) __bool int absl::lts_2020_02_25::random_internal::HasRandenHwAesImplementation()':
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:526:6: error: ambiguating new declaration of '__vector(4) __bool int absl::lts_2020_02_25::random_internal::HasRandenHwAesImplementation()'
 bool HasRandenHwAesImplementation() { return true; }
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:20:0:
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.h:44:6: note: old declaration 'bool absl::lts_2020_02_25::random_internal::HasRandenHwAesImplementation()'
 bool HasRandenHwAesImplementation();
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/ccs/proj/stf006/glaser/rapids/grpc/third_party/abseil-cpp/absl/random/internal/randen_hwaes.cc:526:46: error: cannot convert 'bool' to '__vector(4) __bool int' in return
 bool HasRandenHwAesImplementation() { return true; }
                                              ^~~~
make[2]: *** [third_party/abseil-cpp/absl/random/CMakeFiles/absl_random_internal_randen_hwaes_impl.dir/internal/randen_hwaes.cc.o] Error 1
make[1]: *** [third_party/abseil-cpp/absl/random/CMakeFiles/absl_random_internal_randen_hwaes_impl.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

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[jglaser]

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