vga: Fix the support of extended ASCII

VGA defineds the extended ASCII set based on CP437, but there is a bug
on printing them: in vga_write_at_offset(), we perform a bitwise or
between 'unsigned short' and 'signed char':

```
    p[i] = 0x0F00 | string[i];
```

If we want to show an extended ASCII character, string[i] will be
negative and this bitwise operation will fail due to their implicit
casting rule: convert signed char to unsigned short by adding 65536.

To fix this, we need to cast the string to unsigned char manually
somewhere. Since we still want to leverage the built-in string utilities
which only accepts const char*, we still preserve the original
prototypes before, and cast it until we write into the frame buffer.

BRANCH=brya
BUG=b:264666392
TEST=emerge-brya coreboot chromeos-bootimage
     and verify drawing characters with code > 127.

Change-Id: I9cd65fe9794e5b0d338147924f28efd27fc8a1e8
Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73090
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tarun Tuli <taruntuli@google.com>

src/drivers/pc80/vga/vga.c

@@ -267,7 +267,7 @@ vga_write_at_offset(unsigned int line, unsigned int offset, const char *string)
 
 	for (i = 0; i < (VGA_COLUMNS - offset); i++) {
 		if (i < len)
-			p[i] = 0x0F00 | string[i];
+			p[i] = 0x0F00 | (unsigned char)string[i];
 		else
 			p[i] = 0x0F00;
 	}