powerpc/prom_init: Fix build failure with GCC_PLUGIN_STRUCTLEAK_BYREF_ALL and KASAN

When CONFIG_KASAN is selected, we expect prom_init to use __memset()
because it is too early to use memset().

But with CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL, the compiler adds calls
to memset() to clear objects on stack, hence the following failure:

	  PROMCHK arch/powerpc/kernel/prom_init_check
	Error: External symbol 'memset' referenced from prom_init.c
	make[2]: *** [arch/powerpc/kernel/Makefile:204 : arch/powerpc/kernel/prom_init_check] Erreur 1

prom_find_machine_type() is called from prom_init() and is called only
once, so lets put compat[] in BSS instead of stack to avoid that.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3802811f7cf94f730be44688539c01bba3a3b5c0.1654875808.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/prom_init.c

@@ -2302,7 +2302,7 @@ static void __init prom_init_stdout(void)
 
 static int __init prom_find_machine_type(void)
 {
-	char compat[256];
+	static char compat[256] __prombss;
 	int len, i = 0;
 #ifdef CONFIG_PPC64
 	phandle rtas;