Add partial thread safety to GCC

Add lock functions so that malloc and environment variable access are
thread safe.  Add the compiler option "-o thread-safe" to use the full
version of newlib which is thread safe.

Note that this patch does NOT make file access thread safe.

Author: c1728p9

rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h

@@ -624,11 +624,18 @@ __asm void __rt_entry (void) {
 
 #elif defined (__GNUC__)
 
+osMutexDef(malloc_mutex);
+static osMutexId malloc_mutex_id;
+osMutexDef(env_mutex);
+static osMutexId env_mutex_id;
+
 extern void __libc_fini_array(void);
 extern void __libc_init_array (void);
 extern int main(int argc, char **argv);
 
 void pre_main(void) {
+    malloc_mutex_id = osMutexCreate(osMutex(malloc_mutex));
+    env_mutex_id = osMutexCreate(osMutex(env_mutex));
     atexit(__libc_fini_array);
     __libc_init_array();
     main(0, NULL);
@@ -651,6 +658,29 @@ __attribute__((naked)) void software_init_hook (void) {
   );
 }
 
+// Opaque declaration of _reent structure
+struct _reent;
+
+void __malloc_lock( struct _reent *_r )
+{
+    osMutexWait(malloc_mutex_id, osWaitForever);
+}
+
+void __malloc_unlock( struct _reent *_r )
+{
+    osMutexRelease(malloc_mutex_id);
+}
+
+void __env_lock( struct _reent *_r )
+{
+    osMutexWait(env_mutex_id, osWaitForever);
+}
+
+void __env_unlock( struct _reent *_r )
+{
+    osMutexRelease(env_mutex_id);
+}
+
 #elif defined (__ICCARM__)
 
 extern void* __vector_table;

workspace_tools/toolchains/gcc.py

@@ -183,7 +183,8 @@ def __init__(self, target, options=None, notify=None, macros=None, silent=False,
         GCC.__init__(self, target, options, notify, macros, silent, GCC_ARM_PATH, extra_verbose=extra_verbose)
 
         # Use latest gcc nanolib
-        self.ld.append("--specs=nano.specs")
+        if "thread-safe" not in self.options:
+            self.ld.append("--specs=nano.specs")
         if target.name in ["LPC1768", "LPC4088", "LPC4088_DM", "LPC4330", "UBLOX_C027", "LPC2368"]:
             self.ld.extend(["-u _printf_float", "-u _scanf_float"])
         elif target.name in ["RZ_A1H", "VK_RZ_A1H", "ARCH_MAX", "DISCO_F407VG", "DISCO_F429ZI", "DISCO_F469NI", "NUCLEO_F401RE", "NUCLEO_F410RB", "NUCLEO_F411RE", "NUCLEO_F446RE", "ELMO_F411RE", "MTS_MDOT_F411RE", "MTS_DRAGONFLY_F411RE", "DISCO_F746NG"]: