[gdb/testsuite] Use function_range in gdb.dwarf2/dw2-abs-hi-pc.exp

[gdb/testsuite] Use function_range in gdb.dwarf2/dw2-abs-hi-pc.exp

When I run test-case gdb.dwarf2/dw2-abs-hi-pc.exp with gcc, we have:
...
(gdb) break hello^M
Breakpoint 1 at 0x4004c0: file dw2-abs-hi-pc-hello.c, line 24.^M
(gdb) PASS: gdb.dwarf2/dw2-abs-hi-pc.exp: break hello
...
but with clang, I run into:
...
(gdb) break hello^M
Breakpoint 1 at 0x4004e4^M
(gdb) FAIL: gdb.dwarf2/dw2-abs-hi-pc.exp: break hello
...

The problem is that the CU and function both have an empty address range:
...
 <0><d2>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <108>   DW_AT_name        : dw2-abs-hi-pc-hello.c
    <123>   DW_AT_low_pc      : 0x4004e0
    <127>   DW_AT_high_pc     : 0x4004e0
 <1><12f>: Abbrev Number: 2 (DW_TAG_subprogram)
    <131>   DW_AT_name        : hello
    <13a>   DW_AT_low_pc      : 0x4004e0
    <13e>   DW_AT_high_pc     : 0x4004e0
...

The address ranges are set like this in dw2-abs-hi-pc-hello-dbg.S:
...
        .4byte  .hello_start    /* DW_AT_low_pc */
        .4byte  .hello_end      /* DW_AT_high_pc */
...
where the labels refer to dw2-abs-hi-pc-hello.c:
...
extern int v;

asm (".hello_start: .globl .hello_start\n");
void
hello (void)
{
asm (".hello0: .globl .hello0\n");
  v++;
asm (".hello1: .globl .hello1\n");
}
asm (".hello_end: .globl .hello_end\n");
...

Using asm labels in global scope is a known source of problems, as explained
in the comment of proc function_range in gdb/testsuite/lib/dwarf.exp.

Fix this by using function_range instead.

Tested on x86_64-linux with gcc and clang-7 and clang-12.