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 master in 4 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1982887
mingw-build

00:04:09

passed #1982885
shared-build

00:03:57

passed #1982886
static-build

00:03:52