libm/t_errhandling.c: Put __TEST_FENV before any #includes.

This pattern avoids any potential problem with transitive inclusions
of fenv.h. (No change here because nothing else this file includes
t

libm/t_errhandling.c: Put __TEST_FENV before any #includes.

This pattern avoids any potential problem with transitive inclusions
of fenv.h. (No change here because nothing else this file includes
transitively pulls in fenv.h, but being in the habit of doing it this
way avoids a class of problems.)

PR standards/56234: missing C99 frobs in <math.h>

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Add __TEST_FENV define to fix vax build

math.h: Add missing file for math_errhandling test.

PR standards/56234: missing C99 frobs in <math.h>