[gdb/breakpoint, PIE] Handle setting breakpoint on label without address

When adding:
...
if ![runto_main] then {
    fail "can't run to main"
    return 0
}
...
to test-case gdb.base/label-without-address.exp and running it with target
board unix/-fPIE/-pie, we run into:
...
(gdb) break main:L1^M
Breakpoint 2 at 0x555555554000: file label-without-address.c, line 22.^M
...
That is, for a label with optimized-out address, we set a breakpoint at the
relocation base.

The root cause is that the dwarf reader, despite finding that attribute
DW_AT_low_pc is missing, still tags the L1 symbol as having LOC_LABEL, which
means it has a valid address, which defaults to 0.

Fix this by instead tagging the L1 symbol with LOC_OPTIMIZED_OUT.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26546
	* dwarf2/read.c (new_symbol): Tag label symbol without DW_AT_low_pc as
	LOC_OPTIMIZED_OUT instead of LOC_LABEL.

gdb/testsuite/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26546
	* gdb.base/label-without-address.exp: Runto main first.

---
 gdb/dwarf2/read.c                                | 4 +++-
 gdb/testsuite/gdb.base/label-without-address.exp | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 0ac8533263..b37f7e7a2f 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -21447,10 +21447,12 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	      addr = attr->value_as_address ();
 	      addr = gdbarch_adjust_dwarf2_addr (gdbarch, addr + baseaddr);
 	      SET_SYMBOL_VALUE_ADDRESS (sym, addr);
+	      SYMBOL_ACLASS_INDEX (sym) = LOC_LABEL;
 	    }
+	  else
+	    SYMBOL_ACLASS_INDEX (sym) = LOC_OPTIMIZED_OUT;
 	  SYMBOL_TYPE (sym) = objfile_type (objfile)->builtin_core_addr;
 	  SYMBOL_DOMAIN (sym) = LABEL_DOMAIN;
-	  SYMBOL_ACLASS_INDEX (sym) = LOC_LABEL;
 	  add_symbol_to_list (sym, cu->list_in_scope);
 	  break;
 	case DW_TAG_subprogram:
diff --git a/gdb/testsuite/gdb.base/label-without-address.exp b/gdb/testsuite/gdb.base/label-without-address.exp
index 0fcb1fd19a..c688149cf3 100644
--- a/gdb/testsuite/gdb.base/label-without-address.exp
+++ b/gdb/testsuite/gdb.base/label-without-address.exp
@@ -19,6 +19,11 @@ if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
     return -1
 }
 
+if ![runto_main] then {
+    fail "can't run to main"
+    return 0
+}
+
 set supported 0
 gdb_test_multiple "l main:L1" "" {
     -wrap -re "No label \"L1\" defined in function \"main\"\." {