Fix INT8 that was unsigned instead of signed

[flozz]

Since 2016, the char type is unsigned by default in SDCC. From the SDCC ChageLog:

2016-02-01 Philipp Klaus Krause <pkk AT spth.de>

    * src/SDCCglobl.h,
      src/SDCCmain.c,
      src/SDCCsymt.c,
      src/SDCCval.c,
      doc/sdccman.lyx:
      Make char unsigned by default.

• other source: https://www.z88dk.org/forum/viewtopic.php?id=9331

As the INT8 type is based on char, this make it unsigned...

This PR fixes the issue. There is also an other possible fix: using the --fsigned-char option of the compiler.

Here a small code that reproduce the issue:

#include <stdio.h>
#include <gb/gb.h>

void main(void) {
    INT8 x = 10;
    INT8 dx = -1;
    printf("INT8 x = 10;\n");
    printf("INT8 dx = -1;\n\n");
    printf("x + dx = %i\n", x + dx);
}

Result before the fix:

And after the patch:

[andreasjhkarlsson]

Nice find! Thanks a lot!