[gdb/testsuite] Fix gdb.dwarf2/dw2-lines.exp on arm-linux

[gdb/testsuite] Fix gdb.dwarf2/dw2-lines.exp on arm-linux

With test-case gdb.dwarf2/dw2-lines.exp on arm-linux, I run into:
...
(gdb) break bar_label^M
Breakpoint 2 at 0x4004f6: file dw2-lines.c, line 29.^M
(gdb) continue^M
Continuing.^M
^M
Breakpoint 2, bar () at dw2-lines.c:29^M
29        foo (2);^M
(gdb) PASS: $exp: cv=2: cdw=32: lv=2: ldw=32: continue to breakpoint: foo \(1\)
...

The pass is incorrect because the continue lands at line 29 with "foo (2)"
instead of line line 27 with "foo (1)".

A minimal version is:
...
$ gdb -q -batch dw2-lines.cv-2-cdw-32-lv-2-ldw-32 -ex "b bar_label"
Breakpoint 1 at 0x4f6: file dw2-lines.c, line 29.
...
where:
...
000004ec <bar>:
 4ec:	b580      	push	{r7, lr}
 4ee:	af00      	add	r7, sp, #0

000004f0 <bar_label>:
 4f0:	2001      	movs	r0, #1
 4f2:	f7ff fff1 	bl	4d8 <foo>

000004f6 <bar_label_2>:
 4f6:	2002      	movs	r0, #2
 4f8:	f7ff ffee 	bl	4d8 <foo>
...

So, how does this happen?  In short:
- skip_prologue_sal calls arm_skip_prologue with pc == 0x4ec,
- thumb_analyze_prologue returns 0x4f2
  (overshooting by 1 insn, PR tdep/31981), and
- skip_prologue_sal decides that we're mid-line, and updates to 0x4f6.

However, this is a test-case about .debug_line info, so why didn't arm_skip_prologue
use the line info to skip the prologue?

The answer is that the line info starts at bar_label, not at bar.

Fixing that allows us to work around PR tdep/31981.

Likewise in gdb.dwarf2/dw2-line-number-zero.exp.

Instead, add a new test-case gdb.arch/skip-prologue.exp that is dedicated to
checking quality of architecture-specific prologue analysis, without being
written in an architecture-specific way.

If fails on arm-linux for both marm and mthumb:
...
FAIL: gdb.arch/skip-prologue.exp: f2: $bp_addr == $prologue_end_addr (skipped too much)
FAIL: gdb.arch/skip-prologue.exp: f4: $bp_addr == $prologue_end_addr (skipped too much)
...
and passes for:
- x86_64-linux for {m64,m32}x{-fno-PIE/-no-pie,-fPIE/-pie}
- aarch64-linux.

Tested on arm-linux.