Compile ac_math with clang

[nkkav]

A set of changes to make ac_math based code (e.g., the test drivers under tests) to work with clang-8, clang-9 and clang-10 (other clang versions will likely work but are not tested).

Local testing used GCC_EXEC = clang++ and ran all ac_math tests which concluded with

=============================================================================
  Testbench finished. Maximum errors observed across all data type / bit-width variations:
    max_error_fixed       = 0.0994861
    cmplx_max_error_fixed = 2.23468
    max_error_float       = 0.100706
  ac_sqrt_pwl - PASSED
=============================================================================

meaning the last test (as well as all the previous tests) passed.

The changes are known to work a number of g++ versions (all versions tested) but they have not been tested with the EDG compiler frontend.