build: convert NDK path on Windows
build: convert NDK path on Windows

Convert the NDK path to allow the native path to be used when
configuring the Swift runtime but use the CMake spelling when
performing the inner configure.

glessard/swift,ahoppen/swift,glessard/swift,apple/swift,benlangmuir/swift,atrick/swift,benlangmuir/swift,atrick/swift,apple/swift,ahoppen/swift,atrick/swift,apple/swift,glessard/swift,glessard/swift,ahoppen/swift,JGiola/swift,JGiola/swift,roambotics/swift,JGiola/swift,JGiola/swift,roambotics/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,roambotics/swift,glessard/swift,atrick/swift,atrick/swift,JGiola/swift,ahoppen/swift,apple/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,apple/swift,atrick/swift,ahoppen/swift,roambotics/swift,roambotics/swift,glessard/swift