[debugserver] Detect arch from LLVM_DEFAULT_TARGET_TRIPLE

The debugserver build needs to conditionally include files depending on the
target architecture.

Switch on the architecture specified by LLVM_DEFAULT_TARGET_TRIPLE, as
the llvm and swift build systems use this variable to identify the
target (the latter, indirectly, through LLVM_HOST_TRIPLE).

It would be possible to switch on CMAKE_OSX_ARCHITECTURES, but the swift
build does not provide it, preferring instead to pass arch-specific
CFLAGS etc explicitly. Switching on LLVM_HOST_TRIPLE is also an option,
but it breaks down when cross-compiling.

Differential Revision: https://reviews.llvm.org/D69523