Demo/CORTEX_M3_MPS2_QEMU_GCC: Provide picolibc syscall implementation

Instead of a set of POSIX-compatible APIs as needed by newlib,
picolibc needs a FILE struct allocated that references a function to
output a single character. Picolibc also doesn't need sbrk as it has
its own version

Signed-off-by: Keith Packard <keithpac@amazon.com>

FreeRTOS/Demo/CORTEX_M3_MPS2_QEMU_GCC/syscall.c

@@ -50,8 +50,6 @@ extern unsigned long _heap_bottom;
 extern unsigned long _heap_top;
 extern unsigned long g_ulBase;
 
-static void * heap_end = 0;
-
 /**
  * @brief initializes the UART emulated hardware
  */
@@ -61,6 +59,34 @@ void uart_init()
     UART0_ADDR->CTRL = UART_CTRL_TX_EN;
 }
 
+#ifdef __PICOLIBC__
+
+#include <stdio.h>
+
+/**
+ * @brief  Write byte to the UART channel to be displayed on the command line
+ *         with qemu
+ * @param [in] c     byte to send
+ * @param [in] file  ignored
+ * @returns the character written (cast to unsigned so it is not an error value)
+ */
+
+int
+_uart_putc(char c, FILE *file)
+{
+    (void) file;
+    UART_DR( UART0_ADDR ) = c;
+    return (unsigned char) c;
+}
+
+static FILE __stdio = FDEV_SETUP_STREAM(_uart_putc, NULL, NULL, _FDEV_SETUP_WRITE);
+
+FILE *const stdout = &__stdio;
+
+#else
+
+static void * heap_end = 0;
+
 /**
  * @brief not used anywhere in the code
  * @todo  implement if necessary
@@ -132,6 +158,8 @@ void * _sbrk( int incr )
     return prev_heap_end;
 }
 
+#endif
+
 #ifdef __cplusplus
     }
 #endif