8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2

Backport-of: 202c0137b86cd7bcbe0c1eddf2657f45698ab667

Author: fthevenet

make/RunTestsPrebuilt.gmk

@@ -157,6 +157,10 @@ ifeq ($(UNAME_OS), CYGWIN)
   OPENJDK_TARGET_OS := windows
   OPENJDK_TARGET_OS_TYPE := windows
   OPENJDK_TARGET_OS_ENV := windows.cygwin
+else ifeq ($(UNAME_OS), MINGW64)
+  OPENJDK_TARGET_OS := windows
+  OPENJDK_TARGET_OS_TYPE := windows
+  OPENJDK_TARGET_OS_ENV := windows.msys2
 else
   OPENJDK_TARGET_OS_TYPE:=unix
   ifeq ($(UNAME_OS), Linux)
@@ -171,6 +175,9 @@ else
   OPENJDK_TARGET_OS_ENV := $(OPENJDK_TARGET_OS)
 endif
 
+# Sanity check env detection
+$(info Detected target OS, type and env: [$(OPENJDK_TARGET_OS)] [$(OPENJDK_TARGET_OS_TYPE)] [$(OPENJDK_TARGET_OS_ENV)])
+
 # Assume little endian unless otherwise specified
 OPENJDK_TARGET_CPU_ENDIAN := little