Fix backsliding on use of nullptr instead of NULL

Most uses of NULL in HotSpot source were replaced with nullptr, under the umbrella of JDK-8299837. Since then there has been a little bit of backsliding.

./share/cds/filemap.cpp -- new code
./share/prims/whitebox.cpp -- new code
./share/services/heapDumper.cpp -- new code

./cpu/aarch64/c2_MacroAssembler_aarch64.cpp -- new comments
./cpu/s390/templateTable_s390.cpp -- new comments
./cpu/riscv/c2_MacroAssembler_riscv.cpp -- new comments
./os/aix/os_aix.cpp -- new comment
./os/linux/os_linux.cpp -- new comment
./share/runtime/os.hpp -- new comment