Update Darwin : change Archflags to POCO_TARGET_OSARCH

[nickorr]

I was having issues compiling poco for arm64 on an x86 macOS machine, as a static build. The end result was always x86. Change is from :

ARCHFLAGS ?= -arch $(POCO_HOST_OSARCH) to ARCHFLAGS ?= -arch $(POCO_TARGET_OSARCH)

Otherwise this always compiles to the HOST OS instead of target. I think this forces you to set POCO_TARGET_OSARCH on the make command, but I'm not sure how to otherwise set a default.

Also OSFLAGS should be able to be something overwritten from the make command, but not sure how to do that. I was hoping to :

make install -s -j4 POCO_CONFIG=Darwin64-clang-libc++ MACOSX_DEPLOYMENT_TARGET=10.15 POCO_TARGET_OSARCH=arm64

So 10.15 instead of 10.11. Also I think the isysroot part of OSFLAGS can be left out, as it should be part of the default path now.