build: support CMAKE_SWIFT_COMPILER_TARGET

[compnerd]

This adds support for a new variable CMAKE_SWIFT_COMPILER_TARGET which
mirror's CMake's CMAKE_Swift_COMPILER_TARGET.

[ephemer]

I'm a bit hazy on CMake's case sensitivity (which has been an issue for the Android build in this repo before)- is there are reason to spell this differently compared to what CMake expects internally?

[compnerd]

Yes, it is “misspelt” because right now all the builds use the old spelling of CMAKE_SWIFT_COMPILER.

[ephemer]

@compnerd maybe we should add a step that sets CMAKE_SWIFT_COMPILER_TARGET if (only) CMAKE_C_COMPILER_TARGET has been set to avoid breaking everyone's builds

[compnerd]

Well, the use of CMAKE_SWIFT_COMPILER_TARGET is only useful when doing a cross-compile, most people aren’t doing that, so I don’t really think that this would break most people’s build.

[ktopley-apple]

@swift-ci please test

[vgorloff]

FYI: Possible values for CMAKE_SWIFT_COMPILER_TARGET for Android cross-compile builds:

-DCMAKE_SWIFT_COMPILER_TARGET=armv7-none-linux-androideabi
-DCMAKE_SWIFT_COMPILER_TARGET=aarch64-none-linux-android
-DCMAKE_SWIFT_COMPILER_TARGET=i686-none-linux-android
-DCMAKE_SWIFT_COMPILER_TARGET=x86_64-none-linux-android