[librm] Mark virt_offset, text16, data16, rm_cs, and rm_ds as constant

The physical locations of .textdata, .text16 and .data16 are constant
from the point of view of C code.  Mark the relevant variables as
constant to allow gcc to optimise out redundant reads.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/librm.h

 	"call prot_call\n\t"
 
 /* Variables in librm.S */
-extern unsigned long virt_offset;
+extern const unsigned long virt_offset;
 
 /**
  * Convert physical address to user pointer
  *
  */
 
-extern char *data16;
-extern char *text16;
+extern char * const data16;
+extern char * const text16;
 
 #define __data16( variable )						\
 	__attribute__ (( section ( ".data16" ) ))			\
 /* Variables in librm.S, present in the normal data segment */
 extern uint16_t rm_sp;
 extern uint16_t rm_ss;
-extern uint16_t __text16 ( rm_cs );
+extern const uint16_t __text16 ( rm_cs );
 #define rm_cs __use_text16 ( rm_cs )
-extern uint16_t __text16 ( rm_ds );
+extern const uint16_t __text16 ( rm_ds );
 #define rm_ds __use_text16 ( rm_ds )
 
 extern uint16_t copy_user_to_rm_stack ( userptr_t data, size_t size );