scripts/kallsyms: add sym_name() to mitigate cast ugliness

sym_entry::sym is (unsigned char *) instead of (char *) because
kallsyms exploits the MSB for compression, and the characters are
used as the index of token_profit array.

However, it requires casting (unsigned char *) to (char *) in some
places since standard library functions such as strcmp(), strlen()
expect (char *).

Introduce a new helper, sym_name(), which advances the given pointer
by 1 and casts it to (char *).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>