build: fix fpclassify -Wfloat-conversion warning #15570

Work around a glibc bug where it truncates the argument to fpclassify()
from double to float by implementing fpclassify() ourselves.

Correctness test (Note that the FP_SUBNORMAL test depends on an atof() that
knows how to parse subnormals. Glibc does, not sure about other libcs.):

    #include <math.h>
    #include <stdint.h>
    #include <string.h>

    int xfpclassify(double d)
    {
      uint64_t m;
      int e;

      memcpy(&m, &d, sizeof(m));
      e = 0x7ff & (m >> 52);
      m = 0xfffffffffffffULL & m;

      switch (e) {
        default: return FP_NORMAL;
        case 0x000: return m ? FP_SUBNORMAL : FP_ZERO;
        case 0x7ff: return m ? FP_NAN : FP_INFINITE;
      }
    }

    #include <assert.h>
    #include <stdlib.h>

    int main(void)
    {
      assert(FP_ZERO == xfpclassify(atof("0.0")));
      assert(FP_ZERO == xfpclassify(atof("-0.0")));
      assert(FP_NORMAL == xfpclassify(atof("1.0")));
      assert(FP_NORMAL == xfpclassify(atof("-1.0")));
      assert(FP_INFINITE == xfpclassify(atof("inf")));
      assert(FP_INFINITE == xfpclassify(atof("-inf")));
      assert(FP_NAN == xfpclassify(atof("nan")));
      assert(FP_NAN == xfpclassify(atof("-nan")));
      assert(FP_SUBNORMAL == xfpclassify(atof("1.8011670033376514e-308")));
      return 0;
    }

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