Add token '`' to support kernel symbol read

When using "`symbol_name`" in the script, parser will read symbol name
from kernel. Then, the symbol is replaced by a constant number, which
indicates its address in the kernel, or makes the compile fail.

Tested:
test/symbol.kp: Parse symbol names.

Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>