Fix PR symtab/15391

PR symtab/15391 is a failure with the DW_OP_GNU_implicit_pointer
feature.

I tracked it down to a logic error in read_pieced_value.  The code
truncates this_size_bits according to the type size and offset too
early -- it should do it after taking bits_to_skip into account.

This patch fixes the bug.

While testing this, I also tripped across a latent bug because
indirect_pieced_value does not sign-extend where needed.  This patch
fixes this bug as well.

Finally, Pedro pointed out that a previous version implemented sign
extension incorrectly.  This version introduces a new gdb_sign_extend
function for this.  A couple of notes on this function:

* It has the gdb_ prefix to avoid clashes with various libraries that
  felt free to avoid proper namespacing.  There is a "sign_extend"
  function in a Tile GX header, in an SOM-related BFD header (and in
  sh64-tdep.c and as a macro in arm-wince-tdep.c, but those are
  ours...)

* I looked at all the sign extensions in gdb and didn't see ones that
  I felt comfortable converting to use this function; in large part
  because I don't have a good way to test the conversion.

Built and regtested on x86-64 Fedora 18.  New test cases included;
this required a minor addition to the DWARF assembler.  Note that the
DWARF CU made by implptrpiece.exp uses a funny pointer size in order
to show the sign-extension bug on all platforms.

	* dwarf2loc.c (read_pieced_value): Truncate this_size_bits
	after taking bits_to_skip into account.  Sign extend byte_offset.
	* utils.h (gdb_sign_extend): Declare.
	* utils.c (gdb_sign_extend): New function.

	* gdb.dwarf2/implptrpiece.exp: New file.
	* gdb.dwarf2/implptrconst.exp (d): New variable.
	Print d.
	* lib/dwarf2.exp (Dwarf::_location): Handle DW_OP_piece.
testsuite/gdb.base: Make skip test use defined behaviour.

The skip test currently relies on the order of evaluation of
arguments which is not defined. Use the comma operator where
order is defined instead.

gdb/testsuite/ChangeLog:

2013-06-18  Will Newton  <will.newton@linaro.org>

	* gdb.base/skip.c: Use comma to evaluate results of foo()
	and bar() before passing to baz().
	* gdb.base/skip.c: baz() now takes one argument instead of
	two.
	* python/python.c (finish_python_initialization): Decref
	'pythondir' on failure path as well.