[Flang] Adjust preprocessing to build modules correctly

Several module files in .../llvm-project/flang/module check for the
existence of the macro "__x86_64__" to conditionally compile Fortran
code.  Unfortunately, this macro was not being defined anywhere.  This
patch fixes that for compilations targeting 64 bit x86 machines.

I made the following changes --
  -- Removed the test for 32 bit X86 targets.  The rest of the compiler and
  runtime do not support X86 32 bits.
  -- Added predefined macros to define "__x86_64__"  and "__x86__64" to
  be 1 when the target architecture is 64 bit x86 and the "-cpp" option
  is on the command line.
  -- Changed the cmake file for creating the Fortran module files to use the
  "-cpp" option so that the macro "__x86_64__" will be defined when building
  the module files.
  -- Added a test.

Differential Revision: https://reviews.llvm.org/D135810