tests: Rework mocking facility

Using the linker's --wrap feature has the downside that it only covers
references across object files: If foo.c defines a() and b(), with b
calling a, --wrap=a does nothing to that call.

Instead, use objcopy to mark a weak and global so it can be overridden
by another implementation, but only for files originating in src/.

That way mocks - implemented in tests/ - become the source of truth.

TEST=Had such an issue with get_log_level() in a follow-up commit, and
the mock now takes over. Also, all existing unit tests still pass.

Change-Id: I99c6d6e44ecfc73366bf464d9c51c7da3f8db388
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55360
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>

tests/lib/spd_cache-test.c

@@ -42,14 +42,14 @@ static int teardown_spd_cache(void **state)
 }
 
 
-int __wrap_fmap_locate_area_as_rdev(const char *name, struct region_device *area)
+int fmap_locate_area_as_rdev(const char *name, struct region_device *area)
 {
 	return rdev_chain(area, &flash_rdev_rw, 0, flash_buffer_size);
 }
 
 /* This test verifies if load_spd_cache() correctly loads spd_cache pointer and size
-   from provided region_device. Memory region device is returned by
-   __wrap_fmap_locate_area_as_rdev() */
+   from provided region_device. Memory region device is returned by our
+   fmap_locate_area_as_rdev() override. */
 static void test_load_spd_cache(void **state)
 {
 	uint8_t *spd_cache;