Fix prologue analysis for ldr.w and ldrd instruction

Prologue analysis for ldr.w and ldrd instruction obtains offset from
first two bytes that contains opcode of instruction. It should obtain
offset from next two bytes that actually contain operands.

       * arm-tdep.c (thumb_analyze_prologue): Fix offset calculation for
	ldr.w and ldrd instructions.

Signed-off-by: Taimoor Mirza <tmirza@codesourcery.com>