[ios] switch cross32/crosswatch to be executed on x86_64 and use newe…

…r LLVM (#14856)

Contributes to #9621
Fixes #14841

Note that it breaks iOS 32bit on devices. The Apple Watch _might_ still works (only tested on `2019-02`).  It unblocks some build related issues on the Xamarin.iOS `2019-06` integration, and we have to do it anyway. I'll come back to it to fix device related issues.

sdks/builds/ios.mk

@@ -382,12 +382,10 @@ ios-cross32-64_SYSROOT=-isysroot $(XCODE_DIR)/Platforms/MacOSX.platform/Develope
 
 ios-crosswatch_CONFIGURE_FLAGS=--enable-cooperative-suspend
 
-$(eval $(call iOSCrossTemplate,cross32,i386,arm-darwin,target32,llvm36-llvm32,arm-apple-darwin10,$(XCODE32_DIR)))
+$(eval $(call iOSCrossTemplate,cross32,x86_64,arm-darwin,target32,llvm-llvm64,arm-apple-darwin10,$(XCODE32_DIR)))
 $(eval $(call iOSCrossTemplate,cross64,x86_64,aarch64-darwin,target64,llvm-llvm64,aarch64-apple-darwin10,$(XCODE_DIR)))
-$(eval $(call iOSCrossTemplate,crosswatch,i386,armv7k-unknown-darwin,targetwatch,llvm36-llvm32,armv7k-apple-darwin,$(XCODE32_DIR)))
+$(eval $(call iOSCrossTemplate,crosswatch,x86_64,armv7k-unknown-darwin,targetwatch,llvm-llvm64,armv7k-apple-darwin,$(XCODE32_DIR)))
 $(eval $(call iOSCrossTemplate,crosswatch64_32,x86_64,aarch64-apple-darwin10_ilp32,targetwatch64_32,llvm-llvm64,armv7k-apple-darwin_ilp32,$(XCODE_DIR)))
-# 64->arm32 cross compiler
-$(eval $(call iOSCrossTemplate,cross32-64,x86_64,arm-darwin,target32,llvm-llvm64,arm-apple-darwin10,$(XCODE_DIR)))
 
 
 $(ios_FRAMEWORKS_DIR): package-ios-target32 package-ios-target32s package-ios-target64 package-ios-targettv package-ios-targetwatch package-ios-targetwatch64_32 package-ios-sim32 package-ios-sim64 package-ios-simtv package-ios-simwatch $(TOP)/sdks/builds/ios-Mono.framework-Info.plist $(TOP)/sdks/builds/ios-Mono.framework-tvos.Info.plist $(TOP)/sdks/builds/ios-Mono.framework-watchos.Info.plist $(TOP)/sdks/out/ios-target32-$(CONFIGURATION)/lib/libmonosgen-2.0-minversion80.dylib $(TOP)/sdks/out/ios-target32s-$(CONFIGURATION)/lib/libmonosgen-2.0-minversion80.dylib $(TOP)/sdks/out/ios-target64-$(CONFIGURATION)/lib/libmonosgen-2.0-minversion80.dylib