gh-140239: Check for statx on Android (#140395)

Android has Linux's statx, but MACHDEP is "android" on Android, so
configure doesn't check for statx on Android.  Base the check for statx
on ac_sys_system instead, which is "Linux-android" on Android, "Linux"
on other Linux distributions, and "AIX" on AIX (which has an
incompatible function named statx).