fcatomic: Fix EXC_BAD_ACCESS on iOS ARM64

iPhone 2.1 was released a long time ago, and the macro for checking
the target iOS SDK version has changed. We can simplify everything and
do a very basic check.
3 jobs for issue/232 in 5 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1982884
mingw-build

00:05:01

passed #1982882
shared-build

00:05:01

passed #1982883
static-build

00:03:51