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

[gdb/testsuite] Analyze non-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.

Function foo is a leaf function, so add a non-leaf function bar, and step
through it as well (using nexti instead of stepi).

With the updated test-case, on powerpc64le-linux, I run into PR tdep/30049:
...
FAIL: bar: instruction 5: bt 2
FAIL: bar: instruction 5: up
FAIL: bar: instruction 5: [string equal $fid $::main_fid]
FAIL: bar: instruction 6: bt 2
FAIL: bar: instruction 6: up
FAIL: bar: instruction 6: [string equal $fid $::main_fid]
...

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