GDB: Bring back the handling of DW_CC_program

GDB: Bring back the handling of DW_CC_program

Fix a functional regression and restore the handling of DW_CC_program
code of DW_AT_calling_convention attribute for determining the name of
the starting function of the program where the DW_AT_main_subprogram
attribute has not been provided, such as with Fortran code compiled with
GCC versions 4.5.4 and below, or where DWARF version 3 or below has been
requested.  Without it "main" is considered the starting function.  Cf.
GCC PR fortran/43414.

Original code was removed with commit 6209cde4ddb8 ("Delete DWARF
psymtab code"), and then an update to complement commit 81873cc81eff
("[gdb/symtab] Support DW_AT_main_subprogram with -readnow.") has also
been included here.