libphobos: Handle Darwin Arm and AArch64 in fibre context asm.

This code currently fails to build because it contains ELF-
specific directives.  This patch excludes those directives when
the platform is Darwin.

We do not expect switching fibres between threads to be safe here
either owing to the possible caching of TLS pointers.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

libphobos/ChangeLog:

	* libdruntime/config/aarch64/switchcontext.S: Exclude ELF-
	specific constructs for Darwin.
	* libdruntime/config/arm/switchcontext.S: Likewise.
	* libdruntime/core/thread/fiber.d: Disable switching fibres
	between threads.