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

---
 gdb/testsuite/gdb.dwarf2/imported-unit.exp | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/gdb/testsuite/gdb.dwarf2/imported-unit.exp b/gdb/testsuite/gdb.dwarf2/imported-unit.exp
index 25d44874c12..bf535120349 100644
--- a/gdb/testsuite/gdb.dwarf2/imported-unit.exp
+++ b/gdb/testsuite/gdb.dwarf2/imported-unit.exp
@@ -73,24 +73,24 @@ Dwarf::assemble $asm_file {
 	    {name "<artificial>"}
 	} {
 	    imported_unit {
-		{import :$cu_label ref_addr}
+		{import %$cu_label}
 	    }
 	    subprogram {
-		{abstract_origin :$main_label}
+		{abstract_origin %$main_label}
 		{low_pc $main_start addr}
 		{high_pc "$main_start + $main_length" addr}
 	    } {
 		subprogram {
-		    {abstract_origin :$doit_label}
+		    {abstract_origin %$doit_label}
 		    {low_pc $doit_start addr}
 		    {high_pc "$doit_start + $doit_length" addr}
 		} {
 		    formal_parameter {
-			{abstract_origin :$doit_self_label}
+			{abstract_origin %$doit_self_label}
 		    }
 		}
 	        DW_TAG_variable {
-		    {abstract_origin :$foo_label}
+		    {abstract_origin %$foo_label}
 		    {location 4 data1}
 		}
 	    }