[gdb/symtab] Support DW_AT_main_subprogram with -readnow.

DW_AT_main_subprogram is supported in normal mode in read_partial_die, but not
in -readnow mode.

Fix this by adding support for DW_AT_main_subprogram in read_func_scope.

Tested on x86_64-linux with native and RFC target board readnow (
https://sourceware.org/ml/gdb-patches/2019-05/msg00073.html ).

gdb/ChangeLog:

2019-06-10  Tom de Vries  <tdevries@suse.de>

	PR symtab/16264
	PR symtab/24517
	* dwarf2read.c (read_func_scope): Handle DW_AT_main_subprogram.