"REQUIRES: native" means the compiled code needs to be runnable on the
host. Things like JIT and lli may need this.
"REQUIRES: native-arch" (new in this commit) means that the host's
arch needs to be one of the configured targets. Some tools, e.g. llc,
will default to the host triple if none is specified, and will fail if
the host's arch is not included (e.g. building on X86 and only the ARM
target is included).
Adds lit.cfg code to set "native-arch" and modifies all the places
where r243960 added "native" to use "native-arch" instead.
"objects compiled for the *default* target triple will run the host".