[LLVM] adjust lit.cfg.py for Cygwin (llvm#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.

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

@@ -100,7 +100,12 @@ 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.
-if re.search(r"cygwin|windows-gnu|windows-msvc", config.host_triple):
+# 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"]
 
 llc_args = []
@@ -377,10 +382,11 @@ def version_int(ver):
     else:
         config.available_features.add("target-byteorder-little-endian")
 
-if sys.platform in ["win32"]:
+if sys.platform in ["win32", "cygwin"]:
     # ExecutionEngine, no weak symbols in COFF.
     config.available_features.add("uses_COFF")
-else:
+
+if sys.platform not in ["win32"]:
     # Others/can-execute.txt
     config.available_features.add("can-execute")
 
@@ -651,7 +657,7 @@ def host_unwind_supports_jit():
 
 # .debug_frame is not emitted for targeting Windows x64, aarch64/arm64, AIX, or Apple Silicon Mac.
 if not re.match(
-    r"^(x86_64|aarch64|arm64|powerpc|powerpc64).*-(windows-gnu|windows-msvc|aix)",
+    r"^(x86_64|aarch64|arm64|powerpc|powerpc64).*-(windows-cygnus|windows-gnu|windows-msvc|aix)",
     config.target_triple,
 ) and not re.match(r"^arm64(e)?-apple-(macos|darwin)", config.target_triple):
     config.available_features.add("debug_frame")