Fix copy_bitwise()

Fix copy_bitwise()

When the user writes or reads a variable whose location is described
with DWARF pieces (DW_OP_piece or DW_OP_bit_piece), GDB's helper
function copy_bitwise is invoked for each piece.  The implementation of
this function has a bug that may result in a corrupted copy, depending
on alignment and bit size.  (Full-byte copies are not affected.)

This rewrites copy_bitwise, replacing its algorithm by a fixed version,
and adding an appropriate test case.  Without the fix the new test case
fails, e.g.:

  print def_t
  $2 = {a = 0, b = 4177919}
  (gdb) FAIL: gdb.dwarf2/nonvar-access.exp: print def_t

Written in binary, the wrong result above looks like this:

  01111111011111111111111

Which means that two zero bits have sneaked into the copy of the
original all-one bit pattern.  The test uses this simple all-one value
in order to avoid another GDB bug that causes the DWARF piece of a
DW_OP_stack_value to be taken from the wrong end on big-endian
architectures.

gdb/ChangeLog:

	* dwarf2loc.c (extract_bits_primitive): Remove.
	(extract_bits): Remove.
	(copy_bitwise): Rewrite.  Fixes a possible corruption that may
	occur for non-byte-aligned copies.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/nonvar-access.exp: Add a test for accessing
	non-byte-aligned bit fields.