libphobos: Increase size of defaultStackPages on OSX X86_64 targets.

As of macOS 11, libunwind now requires more stack space than 16k, so
default to a larger stack size. This is only applied to X86 as the
PAGESIZE is still 4k, however on AArch64 it is 16k.

libphobos/ChangeLog:

	* libdruntime/core/thread/fiber.d (defaultStackPages): Increase size
	on OSX X86_64 targets.