Add syscall fallbacks to ChibiOS builds (#11573)

* Add fallback syscalls to ChibiOS builds that are present but able to be overridden as appropriate.

* Modified location to be ChibiOS-specific.

tmk_core/chibios.mk

@@ -204,7 +204,8 @@ CHIBISRC = $(STARTUPSRC) \
        $(PLATFORMSRC) \
        $(BOARDSRC) \
        $(STREAMSSRC) \
-       $(CHIBIOS)/os/various/syscalls.c
+       $(CHIBIOS)/os/various/syscalls.c \
+       $(PLATFORM_COMMON_DIR)/syscall-fallbacks.c
 
 # Ensure the ASM files are not subjected to LTO -- it'll strip out interrupt handlers otherwise.
 QUANTUM_LIB_SRC += $(STARTUPASM) $(PORTASM) $(OSALASM) $(PLATFORMASM)

tmk_core/common/chibios/syscall-fallbacks.c

@@ -0,0 +1,91 @@
+/* Copyright 2021 Nick Brassel, QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+
+__attribute__((weak, used)) int _read_r(struct _reent *r, int file, char *ptr, int len) {
+    (void)r;
+    (void)file;
+    (void)ptr;
+    (void)len;
+    return -1;
+}
+
+__attribute__((weak, used)) int _lseek_r(struct _reent *r, int file, int ptr, int dir) {
+    (void)r;
+    (void)file;
+    (void)ptr;
+    (void)dir;
+    return 0;
+}
+
+__attribute__((weak, used)) int _write_r(struct _reent *r, int file, char *ptr, int len) {
+    (void)r;
+    (void)file;
+    (void)ptr;
+    return len;
+}
+
+__attribute__((weak, used)) int _close_r(struct _reent *r, int file) {
+    (void)r;
+    (void)file;
+    return 0;
+}
+
+__attribute__((weak, used)) caddr_t _sbrk_r(struct _reent *r, int incr) {
+    (void)r;
+    (void)incr;
+    return (caddr_t)-1;
+}
+
+__attribute__((weak, used)) int _fstat_r(struct _reent *r, int file, struct stat *st) {
+    (void)r;
+    (void)file;
+    (void)st;
+    return 0;
+}
+
+__attribute__((weak, used)) int _isatty_r(struct _reent *r, int fd) {
+    (void)r;
+    (void)fd;
+    return 1;
+}
+
+__attribute__((weak, used)) void _fini(void) { return; }
+
+__attribute__((weak, used)) pid_t _getpid(void) { return 1; }
+
+__attribute__((weak, noreturn)) void _exit(int i) {
+    (void)i;
+    while (1)
+        ;
+}
+
+__attribute__((weak)) void _kill(void) {}
+
+__attribute__((weak)) void *__dso_handle;
+
+void __cxa_pure_virtual(void);
+
+__attribute__((weak)) void __cxa_pure_virtual() {
+    while (1)
+        ;
+}
+
+#pragma GCC diagnostic pop