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/ksym.kp: Parse kernel symbol names.

Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>
Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
 - Jovi: Use "ksym" instead of symbol, represent kernel symobl reading,
         not userspace symobl.
         Move read_ksym function to util.c
         better error handing when find symobl failure.