llvm/test/ExecutionEngine/MCJIT/load-object-a.ll: Fix not to use %t.cachedir/%p.

%p is like X:\foo\bar.
Whitespace.
lli: LLIObjectCache: Use llvm::sys::path to get dirname.
lli: Tweak CacheName not to contain DOS driveletter.
llvm/test/ExecutionEngine/MCJIT/lit.local.cfg: Add "AMD64" in the host_arch list.

FIXME: We should not take CMake's ${CMAKE_SYSTEM_PROCESSOR}...
llvm/test/ExecutionEngine/MCJIT/load-object-a.ll: Remove "REQUIRES:shell". This doesn't depend on shell's behavior.