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

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

While investigating PR25862 (an assertion failure with target board
cc-with-debug-names), I noticed that the .debug_aranges section in
gdb.dlang/watch-loc.c contains a hardcoded 0x1000:
...
"      .4byte  _Dmain \n"      // Address
"      .4byte  0x1000 \n"      // Length
...

Fix this by using the actual length of _Dmain, along the lines of how that
is done in gdb.mi/dw2-ref-missing-frame-func.c:
...
"      .4byte  _Dmain_end - _Dmain \n" // Length
...
such that the .debug_aranges entry:
...
    Address    Length
    004004a7 0000000b
    00000000 00000000
...
matches the addresses found in the corresponding CU:
...
 <2><fd>: Abbrev Number: 6 (DW_TAG_subprogram)
    <fe>   DW_AT_name        : _Dmain
    <105>   DW_AT_low_pc      : 0x4004a7
    <10d>   DW_AT_high_pc     : 0x4004b2
...

With this fix the assertion failure is no longer triggered for
gdb.dlang/watch-loc.exp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	* gdb.dlang/watch-loc.c (.debug_aranges): Fix _Dmain length.