[arm] Fix fails in dw2-compdir-oldgcc.exp

[arm] Fix fails in dw2-compdir-oldgcc.exp

This patch fixes two fails in dw2-compdir-oldgcc.exp I've seen on arm
target thumb mode.

  FAIL: gdb.dwarf2/dw2-compdir-oldgcc.exp: info source gcc42
  FAIL: gdb.dwarf2/dw2-compdir-oldgcc.exp: info source gcc43

When fill in DW_AT_low_pc, the label should be used rather than the
function, otherwise, the LSB bit of the address in DW_AT_low_pc is
set and the debug info is wrong.  This patch is to add two labels for
functions gcc42 and gcc43 respectively, and use them.  These two
fails are fixed.

gdb/testsuite:

2014-11-22  Yao Qi  <yao@codesourcery.com>

	* gdb.dwarf2/dw2-compdir-oldgcc.S: Define label .Lgcc42_procstart
	and .Lgcc43_procstart.  Use .Lgcc42_procstart instead of gcc42.
	Use .Lgcc43_procstart instead of gcc43.