[patch] Add DW_AT_main_subprogram tag to Fortran main program

[FX]

Attached patch adds the DW_AT_main_subprogram DWARF 4 tag to the main Fortran program.

Bootstrapped and regtested on x86_64-linux. With a recent readelf, a main program compiled with patch says:

 <1><2e>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2f>   DW_AT_name        : (indirect string, offset: 0x5): MAIN__	
    <33>   DW_AT_decl_file   : 1	
    <34>   DW_AT_decl_line   : 1	
    <35>   DW_AT_low_pc      : 0x0	
    <3d>   DW_AT_high_pc     : 0x6	
    <45>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <47>   DW_AT_main_subprogram: 1	
    <47>   DW_AT_calling_convention: 2	(program)


OK to commit?

FX


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