[gdb/testsuite] Fix DW_AT_decl_file in gdb.trace tests

[gdb/testsuite] Fix DW_AT_decl_file in gdb.trace tests

When running gdb.trace/{entry-values.exp,unavailable-dwarf-piece.exp} with
board cc-with-dwz, we run into two failures related to the DW_AT_decl_file
attribute:
- The encoding DW_FOR_sdata is used for DW_AT_decl_file, while the attribute
  is required to have a an "unsigned integer constant" value.
- The DW_AT_decl_file attributes refer to a file with index one, while there's
  no such file.

Fix this by using DW_FOR_udata and the value 0, meaning "no file specified".

Tested on x86_64-linux with board native-gdbserver.

gdb/testsuite/ChangeLog:

2019-06-24  Tom de Vries  <tdevries@suse.de>

	* gdb.trace/entry-values.exp: Use DW_FORM_udata instead of
	DW_FOR_sdata for DW_AT_decl_file.  Use 0 for DW_AT_decl_file.
	* gdb.trace/unavailable-dwarf-piece.exp: Same.