gh-156115: Use the simulator deployment-target flag for iOS simulator builds (#156116)

Ensures that CFLAGS and LDFLAGS use -mios-simulator-version-min, rather
than the device-based -mios-version-min. This only matters if the CFLAGS
and LDFLAGS are used in a bare call to clang without a -target; but that is
a legal usage, so it's worth catching.