hw_x86_64: Declare CPU context offset constants

The constants specify offset values of CPU context member variables as
specified by Genode::Cpu_state [1] and Genode::Cpu::Context [2].

[1] - repos/base/include/x86_64/cpu/cpu_state.h
[2] - repos/base-hw/src/core/include/spec/x86/cpu.h

repos/base-hw/src/core/spec/x86_64/mode_transition.s

@@ -20,6 +20,13 @@
 /* globally mapped buffer storage */
 .set BUFFER_SIZE, 6 * 8
 
+/* offsets of the member variables in a CPU context */
+.set SP_OFFSET,     1 * 8
+.set R8_OFFSET,     2 * 8
+.set RAX_OFFSET,   10 * 8
+.set FLAGS_OFFSET, 18 * 8
+.set CR3_OFFSET,   20 * 8
+
 .section .text
 
 	/*