testsuite/lib: add check_optional_entry for GDBInfoSymbols

testsuite/lib: add check_optional_entry for GDBInfoSymbols

There was already a similar functionality for the GDBInfoModuleSymbols.
This just extends the GDBInfoSymbols.  We will use this feature in a
later commit to make a testcase less GNU specific and more flexible for
other compilers.

Namely, in gdb.fortran/info-types.exp currenlty
GDBInfoSymbols::check_entry is used to verify and test the output of the
info symbols command.  The test, however was written with gfortran as a
basis and some of the tests are not fair with e.g. ifx and ifort as
they test for symbols that are not actually required to be emitted.  The
lines
   GDBInfoSymbols::check_entry "${srcfile}" "" "${character1}"
and
   GDBInfoSymbols::check_entry "${srcfile}" "37" "Type s1;"

check for types that are either not used in the source file (character1)
or should not be emitted by the compiler at global scope (s1) thus no
appearing in the info symbols command.  In order to fix this we will
later use the newly introduced check_optional_entry over check_entry.