Factor common userland and baremetal C functions

This allows add.c to run unmodified on both!

For that to work, use int main on baremetal, and pass the return value to
the final exit.

baremetal/lib/syscalls.c

@@ -0,0 +1,88 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+
+enum {
+    UART_FR_RXFE = 0x10,
+};
+
+#define UART_DR(baseaddr) (*(unsigned int *)(baseaddr))
+#define UART_FR(baseaddr) (*(((unsigned int *)(baseaddr))+6))
+
+int _close(int file) { return -1; }
+
+int _fstat(int file, struct stat *st) {
+    st->st_mode = S_IFCHR;
+    return 0;
+}
+
+int _isatty(int file) { return 1; }
+int _lseek(int file, int ptr, int dir) { return 0; }
+int _open(const char *name, int flags, int mode) { return -1; }
+
+int _read(int file, char *ptr, int len) {
+    int todo;
+    if (len == 0)
+        return 0;
+    while (UART_FR(UART0_ADDR) & UART_FR_RXFE);
+    *ptr++ = UART_DR(UART0_ADDR);
+    for (todo = 1; todo < len; todo++) {
+        if (UART_FR(UART0_ADDR) & UART_FR_RXFE) {
+            break;
+        }
+        *ptr++ = UART_DR(UART0_ADDR);
+    }
+    return todo;
+}
+
+char *heap_end = 0;
+caddr_t _sbrk(int incr) {
+    extern char heap_low;
+    extern char heap_top;
+    char *prev_heap_end;
+    if (heap_end == 0) {
+        heap_end = &heap_low;
+    }
+    prev_heap_end = heap_end;
+    if (heap_end + incr > &heap_top) {
+        /* Heap and stack collision */
+        return (caddr_t)0;
+    }
+    heap_end += incr;
+    return (caddr_t)prev_heap_end;
+}
+
+int _write(int file, char *ptr, int len) {
+    int todo;
+    for (todo = 0; todo < len; todo++) {
+        UART_DR(UART0_ADDR) = *ptr++;
+    }
+    return len;
+}
+
+/* Only 0 is supported for now, arm semihosting cannot handle other values. */
+void _exit(int status) {
+#if defined(GEM5)
+#if defined(__arm__)
+	__asm__ __volatile__ ("mov r0, #0; mov r1, #0; .inst 0xEE000110 | (0x21 << 16);");
+#elif defined(__aarch64__)
+	__asm__ __volatile__ ("mov x0, #0; .inst 0XFF000110 | (0x21 << 16);");
+#endif
+#else
+#if defined(__arm__)
+    __asm__ __volatile__ ("mov r0, #0x18; ldr r1, =#0x20026; svc 0x00123456");
+#elif defined(__aarch64__)
+	/* TODO actually use the exit value here, just for fun. */
+    __asm__ __volatile__ (
+		"mov x1, #0x26\n" \
+		"movk x1, #2, lsl #16\n" \
+		"str x1, [sp,#0]\n" \
+		"mov x0, #0\n" \
+		"str x0, [sp,#8]\n" \
+		"mov x1, sp\n" \
+		"mov w0, #0x18\n" \
+		"hlt 0xf000\n"
+    );
+#endif
+#endif
+}