[gdb/testsuite] Fix .debug_aranges in dw2-case-insensitive-debug.S

[gdb/testsuite] Fix .debug_aranges in dw2-case-insensitive-debug.S

When running gdb.dwarf2/dw2-case-insensitive.exp with target board
cc-with-debug-names, we run into:
...
FAIL: gdb.dwarf2/dw2-case-insensitive.exp: regexp case-sensitive off \
  (GDB internal error)
...
in more detail:
...
(gdb) info functions fUnC_lang
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 dw2-case-insensitive-debug.S
contains a debug_info_offset which is set to 0:
...
        .section        .debug_aranges,"",@progbits
        .4byte  .Laranges_end - .Laranges_start
.Laranges_start:
        .2byte  0x2
        .4byte  0
...
while the compilation unit at offset 0 in the .debug_section of the executable
is in fact not the compilation unit generated from
dw2-case-insensitive-debug.S.

This happens when linked in object files contain dwarf info and are placed in
the .debug_info section before the compilation unit generated from
dw2-case-insensitive-debug.S.

Fix this by defining the debug_info_offset field using the label .Lcu1_begin
that defines the start of the compilation unit:
...
-       .4byte  0
+       .4byte  .Lcu1_begin
...

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.dwarf2/dw2-case-insensitive-debug.S: Fix debug_info_offset in
	.debug_aranges section.