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/mips32.h

@@ -147,7 +147,6 @@ int mips32_examine(struct target *target);
 
 int mips32_register_commands(struct command_context *cmd_ctx);
 
-int mips32_invalidate_core_regs(struct target *target);
 int mips32_get_gdb_reg_list(struct target *target,
 		struct reg **reg_list[], int *reg_list_size);