[gdb/testsuite] Add label option to proc cu

We can use current dwarf assembly infrastructure to declare a label that marks
the start of the CU header:
...
  declare_labels header_start_cu_a
  _section ".debug_info"
  header_start_cu_a : cu {} {
  }
  _section ".debug_info"
  header_start_cu_b : cu {} {
  }
...
on the condition that we switch to the .debug_info section before, which makes
this style of use fragile.

Another way to achieve the same is to use the label as generated by the cu
proc itself:
...
  variable _cu_label
  cu {} {
  }
  set header_start_cu_a $_cu_label
  cu {} {
  }
  set header_start_cu_b $_cu_label
...
but again that seems fragile given that adding a new CU inbetween will
silently result in the wrong value for the label.

Add a label option to proc cu such that we can simply do:
...
  cu { label header_start_cu_a } {
  }
  cu { label header_start_cu_b } {
  }
...

Tested on x86_64-linux.

gdb/testsuite/lib/dwarf.exp

@@ -1258,6 +1258,10 @@ namespace eval Dwarf {
     #                default = default
     # fission 0|1  - boolean indicating if generating Fission debug info
     #                default = 0
+    # label <label>
+    #              - string indicating label to be defined at the start
+    #                of the CU header.
+    #                default = ""
     # BODY is Tcl code that emits the DIEs which make up the body of
     # the CU.  It is evaluated in the caller's context.
     proc cu {options body} {
@@ -1278,6 +1282,7 @@ namespace eval Dwarf {
 	set _cu_is_fission 0
 	set section ".debug_info"
 	set _abbrev_section ".debug_abbrev"
+	set label ""
 
 	foreach { name value } $options {
 	    set value [uplevel 1 "subst \"$value\""]
@@ -1286,6 +1291,7 @@ namespace eval Dwarf {
 		version { set _cu_version $value }
 		addr_size { set _cu_addr_size $value }
 		fission { set _cu_is_fission $value }
+		label { set label $value }
 		default { error "unknown option $name" }
 	    }
 	}
@@ -1318,6 +1324,11 @@ namespace eval Dwarf {
 	set start_label [_compute_label "cu${cu_num}_start"]
 	set end_label [_compute_label "cu${cu_num}_end"]
 
+	if { $label != "" } {
+	    upvar $label my_label
+	    set my_label $_cu_label
+	}
+
 	define_label $_cu_label
 	if {$is_64} {
 	    _op .4byte 0xffffffff