[LLVM] adjust lit.cfg.py for Cygwin (#151416)
Cygwin is like Windows in that it uses COFF, and doesn't emit
.debug_frame on 64-bit architectures.
However, if -elf is appended to the target triple on Cygwin MCJIT remote
tests fail due to `__register_frame` not being defined. Only one test
fails without -elf that succeeds with it, so mark just that test as
XFAIL on Cygwin.
