argp: Expand argp_usage, _option_is_short, and _option_is_end

argp: Expand argp_usage, _option_is_short, and _option_is_end

The argp code uses macro redefinitions to avoid duplicating static inline
implementations for argp_usage, _option_is_short, and _option_is_end.
However, this causes build issues with clang, as some function prototypes
are redefined to add the hidden attribute with libc_hidden_proto.

To avoid extensive changes to internal headers, just expand the function
implementations and avoid the macro redefine tricks.