Re-introduce '_start' labels and add alignment in dw2-dir-file-name test case.

Re-introduce '_start' labels and add alignment in dw2-dir-file-name test case.

On ppc64-linux a function symbol does not point to code, but to the
function descriptor.  Thus the previous change for this test case
broke it:

      https://sourceware.org/ml/gdb-patches/2014-01/msg00275.html

This patch reverts to the original method, re-introducing '_start'
symbols.  In addition, it adds sufficient alignment before the label,
such that the label never points into an alignment gap.

gdb/testsuite/ChangeLog:
	* gdb.dwarf2/dw2-dir-file-name.c (FUNC): Insert alignment and
	define "*_start" label.  Make "name" static.
	* gdb.dwarf2/dw2-dir-file-name.exp: Replace references to
	${name} by references to ${name}_start.