disable ubsan null checks with gcc (#93) #515

Job	Run time
buildifier	13s
build (--config=clang-format)	3m 59s
build (--config=clang-tidy)	10m 20s
coverage	7m 59s
build (--config=verbose-clang-tidy)	11m 16s
test (gcc)	3m 58s
build (--compilation_mode=opt)	4m 2s
test (gcc, asan)	3m 31s
test (gcc, tsan)	3m 26s
test (gcc, ubsan)	4m 22s
test (clang)	3m 3s
test (clang, asan)	3m 23s
test (clang, tsan)	3m 12s
test (clang, ubsan)	3m 0s
	1h 5m 44s

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