[test] Skip some tests on Windows only (#95095)

These tests pass on Linux using lit's internal shell.

Author: jayfoad

llvm/test/DebugInfo/symbolize-gnu-debuglink-fallback.test

@@ -1,4 +1,4 @@
-# REQUIRES: shell
+# UNSUPPORTED: system-windows
 # Ensures that .debuglink can fallback to a separate location. This is normally
 # /usr/lib/debug (or /usr/libdata/debug for NetBSD), but can be configured on
 # the command line (mainly for testing).

llvm/test/tools/llvm-cov/gcov/basic.test

@@ -3,7 +3,7 @@
 
 # Test fails on Windows where internal shell is used due to path separator
 # mismatches.
-REQUIRES: shell
+UNSUPPORTED: system-windows
 
 RUN: rm -rf %t
 RUN: mkdir %t

llvm/test/tools/llvm-rc/windres-preproc.test

@@ -1,7 +1,6 @@
 ;; Some quoted arguments below don't work properly on Windows when llvm-lit
-;; invokes the cmd shell to run the commands. Just require running in a
-;; posix shell, to keep being able to test this corner case on Unix at least.
-; REQUIRES: shell
+;; invokes the cmd shell to run the commands.
+; UNSUPPORTED: system-windows
 
 ; RUN: llvm-windres -### --include-dir %p/incdir1 --include %p/incdir2 "-DFOO1=\\\"foo bar\\\"" -UFOO2 -D FOO3 --preprocessor-arg "-DFOO4=\\\"baz baz\\\"" -DFOO5=\"bar\" %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK1
 ; RUN: llvm-windres -### --include-dir %p/incdir1 --include %p/incdir2 "-DFOO1=\"foo bar\"" -UFOO2 -D FOO3 --preprocessor-arg "-DFOO4=\"baz baz\"" "-DFOO5=bar" %p/Inputs/empty.rc %t.res --use-temp-file | FileCheck %s --check-prefix=CHECK1