gdb/fortran: Allow Flang MAIN_ in Fortran testing

gdb/fortran: Allow Flang MAIN_ in Fortran testing

Name of fortran main function for Flang compiler is MAIN_ while
for gfortran it is MAIN__ . In test cases MAIN__ is hardcoded for
the purpose of inserting breakpoint.

New proc is added to detect main function name depending on the
compiler used.
Fortran specific version of runto_main named fortran_runto_main
is added.

This commit adds support for Flang main function, there should be
no change for gfortran.

gdb/testsuite/ChangeLog

	* lib/fortran.exp (fortran_main): New Proc, handle flang MAIN_,
	(fortran_runto_main): New Proc, fortran version of runto_main.
	* gdb.fortran/array-bounds-high.exp: Handle flang MAIN_.
	* gdb.fortran/array-bounds.exp: Likewise.
	* gdb.fortran/array-slices.exp: Likewise.
	* gdb.fortran/block-data.exp: Likewise.
	* gdb.fortran/charset.exp: Likewise.
	* gdb.fortran/common-block.exp: Likewise.
	* gdb.fortran/complex.exp: Likewise.
	* gdb.fortran/derived-type-function.exp: Likewise.
	* gdb.fortran/derived-type.exp: Likewise.
	* gdb.fortran/info-modules.exp: Likewise.
	* gdb.fortran/info-types.exp: Likewise.
	* gdb.fortran/intrinsics.exp: Likewise.
	* gdb.fortran/library-module.exp: Likewise.
	* gdb.fortran/logical.exp: Likewise.
	* gdb.fortran/max-depth.exp: Likewise.
	* gdb.fortran/module.exp: Likewise.
	* gdb.fortran/multi-dim.exp: Likewise.
	* gdb.fortran/nested-funcs.exp: Likewise.
	* gdb.fortran/print-formatted.exp: Likewise.
	* gdb.fortran/print_type.exp: Likewise.
	* gdb.fortran/printing-types.exp: Likewise.
	* gdb.fortran/ptr-indentation.exp: Likewise.
	* gdb.fortran/ptype-on-functions.exp: Likewise.
	* gdb.fortran/subarray.exp: Likewise.
	* gdb.fortran/vla-alloc-assoc.exp: Likewise.
	* gdb.fortran/vla-datatypes.exp: Likewise.
	* gdb.fortran/vla-history.exp: Likewise.
	* gdb.fortran/vla-ptr-info.exp: Likewise.
	* gdb.fortran/vla-ptype-sub.exp: Likewise.
	* gdb.fortran/vla-ptype.exp: Likewise.
	* gdb.fortran/vla-sizeof.exp: Likewise.
	* gdb.fortran/vla-type.exp: Likewise.
	* gdb.fortran/vla-value-sub-arbitrary.exp: Likewise.
	* gdb.fortran/vla-value-sub-finish.exp: Likewise.
	* gdb.fortran/vla-value-sub.exp: Likewise.
	* gdb.fortran/vla-value.exp: Likewise.
	* gdb.fortran/whatis_type.exp: Likewise.
	* gdb.mi/mi-var-child-f.exp: Likewise.