[gdb/testsuite] Fix .debug_aranges in watch-loc.c

[gdb/testsuite] Fix .debug_aranges in watch-loc.c

When running gdb.dlang/watch-loc.exp with target board cc-with-debug-names, we
run into:
...
FAIL: gdb.dlang/watch-loc.exp: disassemble _Dmain (GDB internal error)
...
in more detail:
...
(gdb) disassemble _Dmain
gdb/dwarf2read.c:5293: internal-error: \
  compunit_symtab* dw2_find_pc_sect_compunit_symtab(objfile*, \
  bound_minimal_symbol, CORE_ADDR, obj_section*, int): \
  Assertion `result != NULL' failed.
...

The problem is that the .debug_aranges section in watch-loc.c contains a
debug_info_offset which is set to 0:
...
asm (
  "      .pushsection    .debug_aranges,\"\",%progbits\n"
  "      .4byte  .Laranges_end - .Laranges_start\n"
  ".Laranges_start:\n"
  "      .2byte  0x2\n"
  "      .4byte  0\n"
...
while the compilation unit at offset 0 in the .debug_section in the executable
is in fact not the compilation unit generated from watch-loc-dw.S.

[ Note: this is a non-trivial test-case.  The file watch-loc-dw.S contains a
.debug_info section, but not an .debug_aranges section or any actual code.
The file watch-loc.c contains code and a .debug_aranges section, but no other
debug section.  So, the intent for the .debug_aranges section in watch-loc.c
is to refer to a compilation unit in the .debug_info section in
watch-loc-dw.S. ]

This happens when linked in object files contain dwarf info and are placed in
the .debug_info section before the compilation units generated from
watch-loc.c and watch-loc-dw.S.

Fix this by defining the debug_info_offset field using a label .Lcu1_begin
that defines the start of an empty .debug_section compilation unit:
...
 asm (
+  "      .pushsection    .debug_info,\"\",%progbits\n"
+  ".Lcu1_begin:"
+  "      .popsection\n"
   "      .pushsection    .debug_aranges,\"\",%progbits\n"
   "      .4byte  .Laranges_end - .Laranges_start \n"
   ".Laranges_start:\n"
   "      .2byte  0x2\n"
-  "      .4byte  0\n"
+  "      .4byte  .Lcu1_begin\n"
...
which during linking merges with the start of the .debug_info section of
watch-loc-dw.S.

Tested on x86_64-linux with native, cc-with-gdb-index and cc-with-debug-names.

gdb/testsuite/ChangeLog:

2019-05-07  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24522
	* gdb.dlang/watch-loc.c: Fix debug_info_offset in .debug_aranges
	section.