Add REQUIRES: arm-registered-target to test using an arm-apple-ios triple.
Add REQUIRES: arm-registered-target to test using an arm-apple-ios triple.

Else, the test fails in LLVM_TARGETS_TO_BUILD=X86 builds like so:
bin/llvm-mc: : error: unable to get target for 'arm64-apple-ios7.0.0'


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@327233 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm