qemu/mcu/arm/startup: Add an implementation for "abort".

This commit adds a naive implementation of the "abort" standard C
library, that is needed by certain C++ runtimes let code link.

Although the Arm toolchain used in the CI image does not need this,
newer or different toolchains may actually need this (eg. the Arm EABI
toolchain provided by Arch Linux).  Given the limited scope of the QEMU
port, the function simply spins forever.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

ports/qemu/mcu/arm/startup.c

@@ -139,6 +139,12 @@ void exit(int status) {
     }
 }
 
+void abort(void) {
+    for (;;) {
+    }
+    MP_UNREACHABLE;
+}
+
 #ifndef NDEBUG
 void __assert_func(const char *file, int line, const char *func, const char *expr) {
     (void)func;