add comment and XFAIL of cygwin target

Author: jeremyd2019

llvm/test/ExecutionEngine/MCJIT/stubs-sm-pic.ll

@@ -1,5 +1,7 @@
 ; RUN: %lli -jit-kind=mcjit -disable-lazy-compilation=false -relocation-model=pic -code-model=small %s
-; XFAIL: target={{(mips|mipsel)-.*}}, target={{(i686|i386).*}}, target={{(aarch64|arm).*}}
+; XFAIL: target={{(mips|mipsel)-.*}}, target={{(i686|i386).*}}, target={{(aarch64|arm).*}}, target={{.*-(cygwin|windows-cygnus)}}
+; This test segfaults on cygwin, but succeeds with cygwin-elf.  Unfortunately,
+; cygwin-elf breaks the remote tests due to lack of __register_frame.
 
 define i32 @main() nounwind {
 entry:

llvm/test/lit.cfg.py

@@ -107,6 +107,11 @@ def get_asan_rtlib():
 # we don't support COFF in MCJIT well enough for the tests, force ELF format on
 # Windows.  FIXME: the process target triple should be used here, but this is
 # difficult to obtain on Windows.
+# Cygwin is excluded from this workaround, even though it is COFF, because this
+# breaks remote tests due to not having a __register_frame function.  The only
+# test that succeeds with cygwin-elf but fails with cygwin is
+# test/ExecutionEngine/MCJIT/stubs-sm-pic.ll so this test is marked as XFAIL
+# for cygwin targets
 if re.search(r"windows-gnu|windows-msvc", config.host_triple):
     lli_args = ["-mtriple=" + config.host_triple + "-elf"]