In langtools/make/Makefile the settings for DEV_NULL are the wrong way round for Cygwin and MKS.
For reference, see jdk/make/common/shared/Platform.gmk, line 297...
# Where is unwanted output to be delivered?
# MKS uses the special file "NUL", cygwin uses the customary unix file.
DEV_NULL = /dev/null
DEV_NULL = NUL