arch/x86/postcar: Use a separate stack for C execution

Add a stack in .bss for C execution. This will make it easier to move
the setup of MTRRs in C code.

Change-Id: I67cbc988051036b1a0519cec9ed614acede31fd7
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54298
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/drivers/intel/fsp1_1/exit_car.S

@@ -7,7 +7,7 @@ chipset_teardown_car:
 
 	pop	%ebx
 	/* Move the stack pointer to real RAM */
-	movl	post_car_stack_top, %esp
+	movl	_estack, %esp
 	/* Align the stack 16 bytes */
 	andl	$0xfffffff0, %esp