gdb: detect main function even when there's no matching msymbol

Currently, GDB will only stop the backtrace at the main function if
there is a minimal symbol with the matching name.  In Fortran programs
compiled with gfortran this is not the case.  The main function is
present in the DWARF, and as marked as DW_AT_main_subprogram, but
there's no minimal symbol.

This commit extends `inside_main_func` to check the full symbols if no
matching minimal symbol is found.

There's an updated test case that covers this change.

gdb/ChangeLog:

	* frame.c (inside_main_func): Check full symbols as well as
	minimal symbols.

gdb/testsuite/ChangeLog:

	* gdb.fortran/mixed-lang-stack.exp (run_tests): Update expected
	output of backtrace.