Redefine NULL to nullptr

Redefine NULL to nullptr

Bootstrap with musl libc fails with numerous "missing sentinel in
function call" errors.  This is because musl defines NULL as 0L for C++,
but gcc requires sentinel value to be a pointer or __null.

Jonathan Wakely says:

    To be really safe during stage 1, GCC should not use NULL as a
    pointer sentinel in C++ code anyway.

    The bootstrap compiler could define it to 0 or 0u, neither of which
    is guaranteed to be OK to pass as a varargs sentinel where a null
    pointer is expected.  Any of (void*)0 or (void*)NULL or nullptr
    would be safe.

While it is possible to fix this by replacing NULL sentinels with
nullptrs, such approach would generate backporting conflicts, therefore
simply redefine NULL to nullptr at the end of system.h, where it would
not confuse system headers.

gcc/ChangeLog:

2020-06-30  Ilya Leoshkevich  <iii@linux.ibm.com>

	PR bootstrap/95700
	* system.h (NULL): Redefine to nullptr.