Two small tweaks for cross compiling

[jeroen]

Two more tiny tweaks that will smoothen the cross compile process:

First the arch detection in libtbb relies on /usr/sbin/sysctl which does not work on linux or macos-cross. As a result it ends up falling back on arch=ia32 which is completely wrong.

RcppParallel/src/tbb/build/macos.inc

Lines 30 to 50 in d4573c1

	ifndef arch
	ifeq ($(shell /usr/sbin/sysctl -n hw.machine),Power Macintosh)
	ifeq ($(shell /usr/sbin/sysctl -n hw.optional.64bitops),1)
	export arch:=ppc64
	else
	export arch:=ppc32
	endif
	else
	ifeq ($(shell /usr/sbin/sysctl -n hw.machine),arm64)
	export arch:=arm64
	else
	ifeq ($(shell /usr/sbin/sysctl -n hw.optional.x86_64 2>/dev/null),1)
	export arch:=intel64
	else
	export arch:=ia32
	endif
	endif
	endif
	endif

The robust way to detect the target arch on Mac is uname -m (the cross environments shim uname to return the target). So we pre-set that in the RcppParallel Makevars, the same way as is done for Windows.

And one final tweak: we introduce a variable TBB_LINK_LIB that we can use to set the location of the target libtbb, without affecting the runtime TBB. If the variable is unset, the current behavior remains the same.

This is needed, because when we cross compile packages that link against RcppParallel, we need have two copies of RcppParallel: one on the host architecture, that is use to load RcppParallel in order to call RcppParallel::RcppParallelLibs(). And then we need a separate installation of RcppParallel for the cross target architecture, that the package we are building should link against.

Currently we have no way of doing this: if we set TBB_LIB to the cross target arch, the RcppParallel won't load, but if we set it to the host (or we leave it undefined) the path returned by RcppParallel::RcppParallelLibs() returns the host architecture and gives a linking error for the cross build.

[kevinushey]

LGTM; thanks!