fix incorrect processing of register and auto storage-classes

[bust6k]

General

In parse.c:read_decl_spec added check if register or auto storage-class specifiers at global scope,then throw a warning notice about.

Standard proof

The behavior is registered by C11 6.9.2.Semantics:

2. A declaration of an identifier for an object that has file scope without an initializer, and
without a storage-class specifier or with the storage-class specifier static, constitutes a
tentative definition.

here's register and auto don't as mentioned as tentative definition

Tests

• test/register.c — warns on register int x = 0;
• test/auto.c — warns on auto int x = 0;