Add instruction to suppress SIGUSR1 in Posix with LLDB debugger (#1245)

While using the macOS default LLDB debugger, a call to vTaskEndScheduler results in an unhandled SIGUSR1 (aka SIGRESUME) when restoring the scheduler thread's signals with pthread_sigmask. This crashes the program.

Added instructions in portable/ThirdParty/GCC/Posix/port.c to suppress SIGUSR1 to prevent LLDB debugger interference when exiting xPortStartScheduler

Thanks to: @johnboiles for pointing it out in #1224

portable/ThirdParty/GCC/Posix/port.c

@@ -48,6 +48,11 @@
 * stdio (printf() and friends) should be called from a single task
 * only or serialized with a FreeRTOS primitive such as a binary
 * semaphore or mutex.
+* 
+* Note: When using LLDB (the default debugger on macOS) with this port, 
+* suppress SIGUSR1 to prevent debugger interference. This can be
+* done by adding the following line to ~/.lldbinit:
+* `process handle SIGUSR1 -n true -p true -s false`
 *----------------------------------------------------------*/
 #ifdef __linux__
     #define _GNU_SOURCE