selftests/nolibc: add mmap_munmap_good test case

mmap() a file with a good offset and then munmap() it. a non-zero offset
is passed to test the 6th argument of my_syscall6().

Note, it is not easy to find a unique file for mmap() in different
scenes, so, a file list is used to search the right one:

- /dev/zero: is commonly used to allocate anonymous memory and is likely
  present and readable

- /proc/1/exe: for 'run' and 'run-user' target, 'run-user' can not find
  '/proc/self/exe'

- /proc/self/exe: for 'libc-test' target, normal program 'libc-test' has
  no permission to access '/proc/1/exe'

- argv0: the path of the program itself, let it pass even with worst
  case scene: no procfs and no /dev/zero

Suggested-by: Willy Tarreau <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Suggested-by: Thomas Weißschuh <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Zhangjin Wu <[email protected]>
Signed-off-by: Willy Tarreau <[email protected]>

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