Add AIX support

[jtroy]

This PR adds AIX support. Note, on AIX:

• madvise() is not functional.
• mlock()/munlock() require root privileges.

Cirrus CI doesn't support AIX, but I have confirmed that the tests pass when given PV_ROOT privileges. Also note that -race isn't supported on aix/ppc64.

bash-4.4$ go test -count=1 -v ./...
=== RUN   TestCycle
--- PASS: TestCycle (0.00s)
=== RUN   TestProtect
--- PASS: TestProtect (0.00s)
=== RUN   TestProtFlags
--- PASS: TestProtFlags (0.00s)
=== RUN   TestGetStartPtr
--- PASS: TestGetStartPtr (0.00s)
PASS
ok      github.com/awnumar/memcall      0.017s

Also, without PV_ROOT, for the sake of completeness:

bash-4.4$ go test -count=1 -v ./...
=== RUN   TestCycle
    memcall_test.go:25: <memcall> could not acquire lock on 0xa00000046c55000, do you have PV_ROOT? [Err: not owner]
    memcall_test.go:36: <memcall> could not free lock on 0xa00000046c55000, do you have PV_ROOT? [Err: not owner]
--- FAIL: TestCycle (0.00s)
=== RUN   TestProtect
--- PASS: TestProtect (0.00s)
=== RUN   TestProtFlags
--- PASS: TestProtFlags (0.00s)
=== RUN   TestGetStartPtr
--- PASS: TestGetStartPtr (0.00s)
FAIL
FAIL    github.com/awnumar/memcall      0.018s
FAIL

This is mostly copy-pasted from the Darwin implementation. I didn't want to muddy the waters, but I'm happy to do some refactoring if you like. Thanks, and let me know if there's anything else you need.

-John

[awnumar]

Thanks for the PR ❤️