gdb.trace: Use manually-defined start labels in unavailable-dwarf-piece.exp

gdb.trace: Use manually-defined start labels in unavailable-dwarf-piece.exp

On powerpc64, foo/bar point to a function descriptor, not to function code.
Since there are no global labels pointing at the actual function code,
let's make our own.

Regression-tested on x86_64.

gdb/testsuite/ChangeLog:

	* gdb.trace/unavailable-dwarf-piece.c (foo): Add foo_start_lbl label.
	(bar): Add bar_start_lbl label.
	* gdb.trace/unavailable-dwarf-piece.exp: Use foo/bar_start_lbl instead
	of foo/bar for emitting DWARF and tracing.