Make test_compiler_info in Fortran tests test the Fortran compiler

[Gary Benson]

Hi all,

Most (all?) of GDB's Fortran tests operate differently depending on
which compiler is used, but the functions to determine which compiler
is in use check the C compiler, not the Fortran compiler.  Running
"make check" with non-matched C and Fortran compilers results in the
failure of most or all of GDB's tests built from Fortran sources.
This series fixes.

Checked on Fedora 32 x86_64, with GNU Fortran as the Fortran compiler,
and with GCC and Clang as the C compiler.  Is this ok to commit?

Cheers,
Gary

--
Gary Benson - he / him / his
Principal Software Engineer, Red Hat