Add basic CMSIS core headers (#384)

Adds a new cmsis_core INTERFACE library, which provides the basic CMSIS macros/definitions, and RP2040.h and system_RP2040.h
By default the isr_ methods in the vector table are renamed to their CMSIS versions, but this can be overridden by defining PICO_CMSIS_RENAME_EXCEPTIONS=0

test/cmsis_test/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(cmsis_test cmsis_test.c)
+
+target_link_libraries(cmsis_test PRIVATE pico_stdlib cmsis_core)
+pico_add_extra_outputs(cmsis_test)

test/cmsis_test/cmsis_test.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+
+#include "RP2040.h"
+#include "pico/stdio.h"
+
+__STATIC_FORCEINLINE int some_function(int i) {
+    return __CLZ(i);
+}
+
+static bool pendsv_called;
+
+void PendSV_Handler(void) {
+    pendsv_called = true;
+}
+
+int main(void) {
+    stdio_init_all();
+    for(int i=0;i<10;i++) {
+        printf("%d %d\n", i, some_function(i));
+    }
+    SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
+    puts(pendsv_called ? "SUCCESS" : "FAILURE");
+}