[build] Don't build Windows host runtimes on Linux (#756)

Fixes the build break from commit baef1ba, which
[broke the `xamarin-android-linux` job][0].

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android-linux/483/

Author: grendello

build-tools/scripts/BuildEverything.mk

@@ -58,10 +58,6 @@ ifneq ($(OS),Linux)
 ALL_HOST_ABIS += \
 	mxe-Win32 \
 	mxe-Win64
-else
-ALL_HOST_ABIS += \
-	linux-Win32 \
-	linux-Win64
 endif
 
 ALL_AOT_ABIS += \