gdb/testsuite: fix skip-tree.exp test for PowerPC

The gdb.base/skip-tree.exp test was added in commit:

  commit 5ed98c177f9a2381fe5c6ab873d05f1e5dcb8f7f
  Date:   Wed Feb 4 20:31:31 2026 +0000

      gdb: introduce '**' for skip globs

This test currently fails on PowerPC due to architectural differences
in function return handling.

On PowerPC64 ELFv2, calls to external functions are followed by a
`nop` instruction that serves as a placeholder for TOC pointer
restoration. Since this `nop` typically has no DWARF line entry,
`finish` reports the call line instead of the next line.

This issue was reported here:

  https://inbox.sourceware.org/gdb-patches/20260415055144.2729164-1-abhay@linux.ibm.com

The fix proposed there was to add a PowerPC specific check so that the
test would send a `next` command after using `finish` to advance to
the expected source line.

This commit actually takes a more general approach.  After calling
`finish` we check which line we ended up on.  If it was the caller
line then we send GDB a `next` to move to the expected line.  If we
arrive directly at the next line, then no `next` is needed.

I've tested this change on x86-64, where no `next` is needed, and
PowerPC, where `next` is now used as required to get the test
passing.

To simplify the pattern matching in the exp file, I tweaked the source
file slightly.  But adding a call to a dummy 'func5' it's now easy to
pattern match as we step through 'func1', 'func2', etc.

Co-Authored-By: Abhay Kandpal <abhay@linux.ibm.com>