Big-endian targets: Don't ignore offset into DW_OP_implicit_value
    
    When a variable's location is expressed as DW_OP_implicit_value, but the
    given value is longer than needed, which bytes should be used?  GDB's
    current logic was introduced with a patch from 2011 and uses the "least
    significant" bytes:
    
      https://sourceware.org/ml/gdb-patches/2011-08/msg00123.html
    
    Now consider a sub-value from such a location at a given offset, accessed
    through DW_OP_implicit_pointer.  Which bytes should be used for that?  The
    patch above *always* uses the last bytes on big-endian targets, ignoring
    the offset.
    
    E.g., given the code snippet
    
      const char foo[] = "Hello, world!";
      const char *a = &foo[0];
      const char *b = &foo[7];
    
    assume that `foo' is described as DW_OP_implicit_value and `a' and `b'
    each as DW_OP_implicit_pointer into that value.  Then with current GDB
    `*a' and `*b' yield the same result -- the string's zero terminator.
    
    This patch basically reverts the portion of the patch above that deals
    with DW_OP_implicit_value.  This fixes the offset handling and also goes
    back to dropping the last instead of the first bytes on big-endian targets
    if the implicit value is longer than needed.  The latter aspect of the
    change probably doesn't matter for actual programs, but simplifies the
    logic.
    
    The patch also cleans up the original code a bit and adds appropriate test
    cases.
    
    gdb/testsuite/ChangeLog:
    
    	* gdb.dwarf2/dw2-op-stack-value.exp: Adjust expected result of
    	taking a 2-byte value out of a 4-byte DWARF implicit value on
    	big-endian targets.
    	* gdb.dwarf2/nonvar-access.exp: Add more comments to existing
    	logic.  Add test cases for DW_OP_implicit.
    
    gdb/ChangeLog:
    
    	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): For
    	DWARF_VALUE_LITERAL, no longer ignore the offset on big-endian
    	targets.  And if the implicit value is longer than needed, extract
    	the first bytes instead of the "least significant" ones.