[gdb] Add gdbarch_dwarf2_reg_piece_offset hook

In rw_pieced_value, when reading/writing part of a register, DW_OP_piece and
DW_OP_bit_piece are handled the same, but the standard tells us:
- DW_OP_piece: if the piece is located in a register, but does not occupy the
  entire register, the placement of the piece within that register is defined
  by the ABI.
- DW_OP_bit_piece: if the location is a register, the offset is from the least
  significant bit end of the register.

Add a new hook gdbarch_dwarf2_reg_piece_offset that allows us to define the
ABI-specific behaviour for DW_OP_piece.

The default implementation of the hook is the behaviour of DW_OP_bit_piece, so
there should not be any functional changes.

Tested on s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/gdbarch_components.py

@@ -829,6 +829,22 @@ allocate and return a struct value with all value attributes
     invalid=False,
 )
 
+Method(
+    comment="""
+For a DW_OP_piece located in a register, but not occupying the
+entire register, return the placement of the piece within that
+register as defined by the ABI.
+""",
+    type="ULONGEST",
+    name="dwarf2_reg_piece_offset",
+    params=[
+        ("int", "regnum"),
+        ("ULONGEST", "size")
+    ],
+    predefault="default_dwarf2_reg_piece_offset",
+    invalid=False,
+)
+
 Method(
     type="CORE_ADDR",
     name="pointer_to_address",