seccomp: move brk+mmap+mmap2 into @default syscall filter set

seccomp: move brk+mmap+mmap2 into @default syscall filter set

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.