These three syscalls are internally used by libc's memory allocation logic, i.e. ultimately back malloc(). Allocating a bit of memory is so basic, it should just be in the default set. This fixes a couple of issues with asan/msan and the seccomp tests: when asan/msan is used some additional, large memory allocations take place in the background, and unless mmap/mmap2/brk are allowlisted these will fail, aborting the test prematurely.
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