[gdb/testsuite] Improve leaf fn in gdb.base/unwind-on-each-insn.exp

[gdb/testsuite] Improve leaf fn in gdb.base/unwind-on-each-insn.exp

In test-case gdb.base/unwind-on-each-insn.exp, we stepi through function foo
to check various invariants at each insn, in particular hoping to excercise
insns that modify stack and frame pointers.

For x86_64-linux, we have a reasonably complex foo (and similar for -m32):
...
  4004bc:       55                      push   %rbp
  4004bd:       48 89 e5                mov    %rsp,%rbp
  4004c0:       90                      nop
  4004c1:       5d                      pop    %rbp
  4004c2:       c3                      ret
...
Both stack pointer (%rsp) and frame pointer (%rbp) are modified.

Also for s390x-linux (and similar for -m31):
...
0000000001000668 <foo>:
 1000668:       e3 b0 f0 58 00 24       stg     %r11,88(%r15)
 100066e:       b9 04 00 bf             lgr     %r11,%r15
 1000672:       e3 b0 b0 58 00 04       lg      %r11,88(%r11)
 1000678:       07 fe                   br      %r14
...
Frame pointer (%r11) is modified.

Likewise for powerpc64le-linux:
...
00000000100006c8 <foo>:
    100006c8:   f8 ff e1 fb     std     r31,-8(r1)
    100006cc:   d1 ff 21 f8     stdu    r1,-48(r1)
    100006d0:   78 0b 3f 7c     mr      r31,r1
    100006d4:   00 00 00 60     nop
    100006d8:   30 00 3f 38     addi    r1,r31,48
    100006dc:   f8 ff e1 eb     ld      r31,-8(r1)
    100006e0:   20 00 80 4e     blr
...
Both stack pointer (r1) and frame pointer (r31) are modified.

But for aarch64-linux, we have:
...
00000000004005fc <foo>:
  4005fc:       d503201f        nop
  400600:       d65f03c0        ret
...
There's no insn that modifies stack or frame pointer.

Fix this by making foo more complex, adding an (unused) argument:
...
0000000000400604 <foo>:
  400604:       d10043ff        sub     sp, sp, #0x10
  400608:       f90007e0        str     x0, [sp, #8]
  40060c:       d503201f        nop
  400610:       910043ff        add     sp, sp, #0x10
  400614:       d65f03c0        ret
...
such that the stack pointer (sp) is modified.

[ Note btw that we're seeing the effects of -momit-leaf-frame-pointer, with
-mno-omit-leaf-frame-pointer we have instead:
...
0000000000400604 <foo>:
  400604:       a9be7bfd        stp     x29, x30, [sp, #-32]!
  400608:       910003fd        mov     x29, sp
  40060c:       f9000fa0        str     x0, [x29, #24]
  400610:       d503201f        nop
  400614:       a8c27bfd        ldp     x29, x30, [sp], #32
  400618:       d65f03c0        ret
...
such that also the frame pointer (x29) is modified. ]

Having made foo more complex, we now run into the following fail on
x86_64/-m32:
...
FAIL: gdb.base/unwind-on-each-insn.exp: instruction 1: $sp_value == $main_sp
....

The problem is that the stack pointer has changed inbetween the sampling of
main_sp and *foo, in particular by the push insn:
...
 804841a:       68 c0 84 04 08          push   $0x80484c0
 804841f:       e8 10 00 00 00          call   8048434 <foo>
...

Fix this by updating main_sp.

On powerpc64le-linux, with gcc 7.5.0 I now run into PR tdep/30021:
...
FAIL: gdb.base/unwind-on-each-insn.exp: insn 7: $fba_value == $main_fba
FAIL: gdb.base/unwind-on-each-insn.exp: insn 7: [string equal $fid $main_fid]
...
but I saw the same failure before this commit with gcc 4.8.5.

Tested on:
- x86_64-linux (-m64 and -m32)
- s390x-linux (-m64 and -m31)
- powerpc64le-linux
- aarch64-linux

Also I've checked that the test-case still functions as regression test for PR
record/16678 on x86_64.