sbsa-acs,edk2-firmware: Adjust target tuple for clang

Current settings assume glibc tuples and hence when building with musl C
library clang can not find correct cross tools and system headers and
libraries, really we should be using exact tuple that is in play so
using HOST_SYS is right replacement for OE

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Jon Mason <jon.mason@arm.com>
Cc: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>