Fix inter-CU references using intra-CU form in imported-unit

Fix inter-CU references using intra-CU form in imported-unit

When running the gdb testsuite with the cc-with-dwz board, I run into:
...
Running gdb/testsuite/gdb.dwarf2/imported-unit.exp ...
gdb compile failed, dwz: gdb.dwarf2/imported-unit/imported-unit: \
  Couldn't find DIE referenced by DW_AT_abstract_origin
cc-with-tweaks.sh: dwz did not modify gdb.dwarf2/imported-unit/imported-unit.
...

The problem is that the DW_AT_abstract_origin reference here:
...
 <0><d2>: Abbrev Number: 2 (DW_TAG_compile_unit)
 <1><e6>: Abbrev Number: 4 (DW_TAG_subprogram)
    <e7>   DW_AT_abstract_origin: <0x142>
    <eb>   DW_AT_low_pc      : 0x4004b2
    <f3>   DW_AT_high_pc     : 0x4004c8
...
referring to a DIE in another compilation unit here:
...
 <0><129>: Abbrev Number: 2 (DW_TAG_compile_unit)
  <1><142>: Abbrev Number: 4 (DW_TAG_subprogram)
    <143>   DW_AT_name        : main
    <148>   DW_AT_type        : <0x13b>
    <14c>   DW_AT_external    : 1
...
is encoded using intra-CU reference form DW_FORM_ref4 instead of intra-CU
reference DW_FORM_ref_addr:
...
   4      DW_TAG_subprogram    [has children]
    DW_AT_abstract_origin DW_FORM_ref4
    DW_AT_low_pc       DW_FORM_addr
    DW_AT_high_pc      DW_FORM_addr
    DW_AT value: 0     DW_FORM value: 0
...

Fix this in the DWARF assembler by making all inter-CU references use the '%'
label prefix.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-12-08  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/imported-unit.exp: Fix inter-CU references.

Change-Id: I690ff18c3943705ed478453531b176ff74700f3c