build: Add back _write stub function

The _write stub was removed by this commit:
https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/29175

If a platform does not using the common uart stdout file and
it is compiled with GCC12+, the missing _write function error
will come up.
Add back the weak definition will solve this issue.

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: I5303433985e7a7e80f14cd22895eb985e99e6092
diff --git a/platform/ext/common/syscalls_stub.c b/platform/ext/common/syscalls_stub.c
index a1c9766..9ebe78e 100644
--- a/platform/ext/common/syscalls_stub.c
+++ b/platform/ext/common/syscalls_stub.c
@@ -48,3 +48,8 @@
 void _read(void)
 {
 }
+
+__attribute__((weak))
+void _write(void)
+{
+}