[librm] Add phys_call() wrapper for calling code with physical addressing

Add a phys_call() wrapper function (analogous to the existing
real_call() wrapper function) for calling code with flat physical
addressing, and use this wrapper within the PHYS_CODE() macro.

Move the relevant functionality inside librm.S, where it more
naturally belongs.

The COMBOOT code currently uses explicit calls to _virt_to_phys and
_phys_to_virt.  These will need to be rewritten if our COMBOOT support
is ever generalised to be able to run in a 64-bit build.
Specifically:

  - com32_exec_loop() should be restructured to use PHYS_CODE()

  - com32_wrapper.S should be restructured to use an equivalent of
    prot_call(), passing parameters via a struct i386_all_regs

  - there appears to be no need for com32_wrapper.S to switch between
    external and internal stacks; this could be omitted to simplify
    the design.

For now, librm.S continues to expose _virt_to_phys and _phys_to_virt
for use by com32.c and com32_wrapper.S.  Similarly, librm.S continues
to expose _intr_to_virt for use by gdbidt.S.

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

src/arch/x86/include/librm.h

@@ -250,11 +250,16 @@ extern void remove_user_from_rm_stack ( userptr_t data, size_t size );
 
 /* PHYS_CODE: declare a fragment of code that executes in flat physical mode */
 #define PHYS_CODE( asm_code_str )			\
-	"call _virt_to_phys\n\t"			\
+	"push $1f\n\t"					\
+	"call phys_call\n\t"				\
+	".section \".text.phys\", \"ax\", @progbits\n\t"\
 	".code32\n\t"					\
+	"\n1:\n\t"					\
 	asm_code_str					\
-	"call _phys_to_virt\n\t"			\
-	CODE_DEFAULT "\n\t"
+	"\n\t"						\
+	"ret\n\t"					\
+	CODE_DEFAULT "\n\t"				\
+	".previous\n\t"
 
 /** Number of interrupts */
 #define NUM_INT 256