[libcpu/arm/cortex-m33]: Fix incorrect CPU architecture in GCC context switch

The context_gcc.S file was incorrectly marked as cortex-m4 in both the
doxygen comment and .cpu directive, while the IAR and KEIL versions
correctly specify cortex-m33. This file uses ARMv8-M specific features
(PSPLIM register, TrustZone support) that are not available in Cortex-M4.

Changes:
- Update doxygen group from cortex-m4 to cortex-m33
- Change .cpu directive from cortex-m4 to cortex-m33

Author: illustriousness

libcpu/arm/cortex-m33/context_gcc.S

@@ -13,13 +13,13 @@
  */
 
 /**
- * @addtogroup cortex-m4
+ * @addtogroup cortex-m33
  */
 /*@{*/
 
 #include <rtconfig.h>
 
-.cpu cortex-m4
+.cpu cortex-m33
 .syntax unified
 .thumb
 .text