_instruction_reserved_start variable

If using `int`, then for CLANG, with enabled optimization when inlined function is provided with the address of external symbol, the two least bits of the constant used inside that function get cleared. Optimizer assumes that address of external symbol should be aligned to 4-bytes and therefore aligns constant value used for bitwise AND operation with that address. This means `extern int _instruction_reserved_start;` can be unaligned to 4 bytes, whereas using `char` can solve this issue. As we only use these symbol address, we declare them as `char` here