slang: change char type to signed char in macros

slang: change char type to signed char in macros

Fix [YOCTO 2726]

C language has 3 distinct char types:
         char
         unsigned char
         signed char
A char has the same range of values as signed char on X86,
but same as unsigned char on ARM which made Slang's typecast()
and array_sort() unable to work for char value on ARM, since
Slang is assuming "char" as "signed char".

Now we change "char" as "signed char" explicitly in
util/mkslarith2.sl, and use it to regenerate src/slarith2.inc

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>