mb/qemu-{i440fx,q35}: Use POSTCAR stage to load the ramstage

Qemu does not have a real CAR but postcar stage is still useful
for testing the stage.

The postcar stage is also mandatory for x86_64 to setup
pagetables for x86_64 ramstage.

Do not set up MTRRs, as qemu ignores them anyways.

Tested on qemu-i440fx and qemu-q35.

Change-Id: I6638534d99fde312e55b6a6be8c95e4cb25cca80
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/30499
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/Makefile.inc

@@ -133,6 +133,7 @@ ramstage-y += wrdd.c
 ramstage-$(CONFIG_CONSOLE_CBMEM) += cbmem_console.c
 ramstage-$(CONFIG_BOOTSPLASH) += jpeg.c
 ramstage-$(CONFIG_TRACE) += trace.c
+postcar-$(CONFIG_TRACE) += trace.c
 ramstage-$(CONFIG_COLLECT_TIMESTAMPS) += timestamp.c
 ramstage-$(CONFIG_COVERAGE) += libgcov.c
 ramstage-y += edid.c