toolchain/nasm: force ar and ranlib only on macOSX

toolchain/nasm: force ar and ranlib only on macOSX

On Debian 9 nasm does not build when we force it to use ranlib, for
macOSX this is needed. Only force this on macOSX and not on any other
OS, this should fix the build of nasm on Linux systems. On my Debian
system the nasm configure script selects  gcc-ranlib and gcc-ar instead.

Fixes: d3a7587eb95 ("toolchain/nasm: fix missing AR/RANLIB variables")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(backported from e204717ef2445fc848b0a70374b03b1c8484d176)