PR symtab/16264 - support DW_AT_main_subprogram

This patch adds support for DW_AT_main_subprogram.
This is PR symtab/16264.

DW_AT_main_subprogram is used to mark a program's entry point.  GCC
can emit this, and I hope to change the Rust compiler to emit it as
well.

GDB already supports an older, pre-DWARF 4 convention adopted by
FORTRAN compilers, namely to emit DW_AT_calling_convention for the
"main" function.  However, I think this support in GDB had a small
bug, in that it seems to rely on the DW_AT_name being read before
DW_AT_calling_convention.  This patch fixes this as well.

Built and regtested on x86-64 Fedora 24 and the buildbot.  New test
case included.

2016-12-02  Tom Tromey  <tom@tromey.com>

	PR symtab/16264:
	* dwarf2read.c (struct partial_die_info) <main_subprogram>: New
	member.
	(add_partial_symbol): Call set_objfile_main_name.
	(read_partial_die): Handle DW_AT_main_subprogram.
	<DW_AT_calling_convention>: don't call set_objfile_main_name, but
	set main_subprogram flag.

2016-12-02  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/main-subprogram.c: New file.
	* gdb.dwarf2/main-subprogram.exp: New file.

1 files changed, 29 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/main-subprogram.c b/gdb/testsuite/gdb.dwarf2/main-subprogram.c
new file mode 100644
index 0000000..97ca22e
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/main-subprogram.c
@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+mymain ()
+{
+  asm ("mymain_label: .globl mymain_label");
+  return 0;
+}
+
+int
+main()
+{
+  return mymain ();
+}