Place all declarations that are local to a function into the local function

Place all declarations that are local to a function into the local function
scope. Apart from some other advantages, this works around a bug where having
an external identifier in global scope and using an "extern" declaration for
exactly the same identifier in a function did not work, because the assembler
refused to export and import one and the same identifier. Since the import
now imports into local scope, both identifiers are distinct for the assembler
and the (valid) C code works.

git-svn-id: svn://svn.cc65.org/cc65/trunk@1424 b7a2c559-68d2-44c3-8de9-860c34a00d81