[clang, test, Darwin] Fix tests expecting Darwin target

[thopre]

Summary

Clang tests Driver/apple-arm64-arch.c and
Driver/darwin-warning-options.c test Darwin driver functionality but
only require the host system to be Darwin. This leads the tests to fail
when building a cross-compiler on Darwin and to be marked unsupported
when cross-compiling to Darwin from another system. This commit changes
the requirements for those tests to require the target to be Darwin.

[steven_wu]

LGTM

clang/test/Driver/apple-arm64-arch.c

 // RUN: env SDKROOT="/" %clang -arch arm64 -c -### %s 2>&1 | \
 // RUN:   FileCheck %s
 //
-// REQUIRES: system-darwin
+// REQUIRES: darwin
 // XFAIL: apple-silicon-mac
 //
 // CHECK: "-triple" "arm64-apple-ios{{[0-9.]+}}"

clang/test/Driver/darwin-warning-options.c

-// REQUIRES: system-darwin
+// REQUIRES: darwin
 
 // Always error about undefined 'TARGET_OS_*' macros on Darwin.
 // RUN: %clang -### %s 2>&1 | FileCheck %s
 
 // CHECK-DAG: "-Wundef-prefix=TARGET_OS_"
 // CHECK-DAG: "-Werror=undef-prefix"