Use qemu_tolower() and qemu_toupper(), not tolower() and toupper()

On NetBSD, where tolower() and toupper() are implemented using an
array lookup, the compiler warns if you pass a plain 'char'
to these functions:

gdbstub.c:914:13: warning: array subscript has type 'char'

This reflects the fact that toupper() and tolower() give
undefined behaviour if they are passed a value that isn't
a valid 'unsigned char' or EOF.

We have qemu_tolower() and qemu_toupper() to avoid this problem;
use them.

(The use in scsi-generic.c does not trigger the warning because
it passes a uint8_t; we switch it anyway, for consistency.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/gdbstub.c b/gdbstub.c
index f936ddd..2a94030 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -911,7 +911,7 @@
 
         cur_action = *p++;
         if (cur_action == 'C' || cur_action == 'S') {
-            cur_action = tolower(cur_action);
+            cur_action = qemu_tolower(cur_action);
             res = qemu_strtoul(p + 1, &p, 16, &tmp);
             if (res) {
                 goto out;