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

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