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/coreboot_table-test.c

@@ -264,7 +264,7 @@ void cbmem_run_init_hooks(int is_recovery)
 }
 
 extern uintptr_t _cbmem_top_ptr;
-void *__wrap_cbmem_top_chipset(void)
+void *cbmem_top_chipset(void)
 {
 	return (void *)_cbmem_top_ptr;
 }