tests: protection: add riscv support

Execute tests are disabled for RISC-V because is isn't able
to set an execution restriction. From RISC-V documentation:
  "Instruction address-translation and protection are unaffected
  by the setting of MPRV"
MPRV is used to apply memory protection restriction when CPU is
running in machine mode (kernel).

Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>

Author: amergnat

tests/kernel/mem_protect/protection/src/main.c

@@ -20,6 +20,11 @@
 #define SKIP_EXECUTE_TESTS
 #endif
 
+/* RISC-V have no mechanism to restrict execution */
+#if defined(CONFIG_RISCV)
+#define SKIP_EXECUTE_TESTS
+#endif
+
 #define INFO(fmt, ...) printk(fmt, ##__VA_ARGS__)
 
 void k_sys_fatal_error_handler(unsigned int reason, const z_arch_esf_t *pEsf)