gdb/testsuite: add Fortran compiler identification to GDB

gdb/testsuite: add Fortran compiler identification to GDB

This commit adds a separate Fortran compiler identification mechanism to
the testsuite, similar to the existing one for C/C++.  Before this
change, the options and version for the Fortran compiler specified when
running the testsuite with F90_FOR_TARGET set, was detected via its
respective C compiler.  So running the testsuite as

  make check TEST=gdb.fortran/*.exp CC_FOR_TARGET=gcc F90_FOR_TARGET=ifx

or even

  make check TEST=gdb.fortran/*.exp F90_FOR_TARGET=ifx

would use the gcc compiler inside the procedures get_compiler_info and
test_compiler_info to identify compiler flags and the compiler version.
This could sometimes lead to unpredictable outputs.  It also limited
testsuite execution to combinations where C and Fortran compiler would
come from the same family of compiers (gcc/gfortran, icc/ifort, icx/ifx,
clang/flang ..).  This commit enables GDB to detect C and Fortran
compilers independently of each other.

As most/nearly all Fortran compilers have a mechanism for preprocessing
files in a C like fashion we added the exact same meachnism that already
existed for C/CXX.  We let GDB preprocess a file with the compilers
Fortran preprocessor and evaluate the preprocessor defined macros in that
file.

This enables GDB to properly run heterogeneous combinations of C and
Fortran compilers such as

  CC_FOR_TARGET='gcc' and F90_FOR_TARGET='ifort'

or enables one to run the testsuite without specifying a C compiler as in

  make check TESTS=gdb.fortran/*.exp F90_FOR_TARGET='ifx'
  make check TESTS=gdb.fortran/*.exp F90_FOR_TARGET='flang'

On the other hand this also requires one to always specify a
identification mechanism for Fortran compilers in the compiler.F90 file.

We added identification for GFORTRAN, FLANG (CLASSIC and LLVM) IFX,
IFORT, and ARMFLANG for now.

Classic and LLVM flang were each tested with their latest releases on
their respective release pages.  Both get recognized by the new compiler
identification and we introduced the two names flang-classic and
flang-llvm to distinguish the two.  While LLVM flang is not quite mature
enough yet for running the testsuite we still thought it would be a good
idea to include it already.  For this we added a case for the fortran_main
procedure.  LLVM flang uses 'MAIN__' as opposed to classic flang which
uses 'MAIN_' here.

We did not have the possibility to test ARMFLANG - the versioning scheme
here was extracted from its latest online documentation.

We changed the test_compiler_info procedure to take another optional
argument, the language string, which will be passed though to the
get_compiler_info procedure.  Passing 'f90' or 'c++' here will then
trigger the C++/Fortran compiler identification within
get_compiler_info.  The latter procedure was extended to also handle
the 'f90' argument (similarly to the already existing 'c++' one).

Co-authored-by: Nils-Christian Kempke <nils-christian.kempke@intel.com>