libgloss: microblaze: fix read and _write declarations

This patch resolves the following build error which occurs with gcc 14.1.0:
../../.././libgloss/microblaze/linux-outbyte.c: In function 'outbyte':
../../.././libgloss/microblaze/linux-outbyte.c:3:9: error: implicit declaration of function '_write' [-Wimplicit-function-declaration]
    3 |         _write(1, &c, 1);

Signed-off-by: Neal Frager <neal.frager@amd.com>

Author: nealfrager

libgloss/microblaze/linux-inbyte.c

@@ -1,3 +1,5 @@
+extern int read (int, char *, int);
+
 int inbyte(void)
 {
 	char ch = 0;

libgloss/microblaze/linux-outbyte.c

@@ -1,3 +1,5 @@
+extern int _write (int, char *, int);
+
 void outbyte (unsigned char c)
 {
 	_write(1, &c, 1);