target: create/use register_cache_invalidate()

Create a generic register_cache_invalidate(), and use it to
replace three all-but-identical core-specific routines:

 - armv4_5_invalidate_core_regs()
 - armv7m_invalidate_core_regs
 - mips32_invalidate_core_regs() too.

Make cache->num_regs be unsigned, avoiding various errors.

Net code shrink and simplification.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/etm.c

@@ -234,7 +234,7 @@ static const struct reg_arch_type etm_scan6_type = {
 static struct reg *etm_reg_lookup(struct etm_context *etm_ctx, unsigned id)
 {
 	struct reg_cache *cache = etm_ctx->reg_cache;
-	int i;
+	unsigned i;
 
 	for (i = 0; i < cache->num_regs; i++) {
 		struct etm_reg *reg = cache->reg_list[i].arch_info;