Fix 32-bit non-SIMD FP regression tests

Fix 32-bit non-SIMD FP regression tests

- Introduce a new FLOATTEST value ("387") on Un*x systems that will
  compare the floating point DCT/IDCT algorithms against the expected
  results from the C algorithms when built using 32-bit code and
  -mfpmath=387.
- Extend the Windows regression tests so that they work properly when
  building libjpeg-turbo with 32-bit code and without SIMD, using either
  Visual C++ (tested with 2008, 2010, 2015) or MinGW.