Add build instructions for System Z s390, too.

[uweigand]

Can you explain why you want to do this?

Since the SystemZ back end at this point does not actually support generating code for the 32-bit s390 target, it seems wrong to accept this as a "native architecture" here ...

[v.g.vassilev]

Adding the author in the loop...

[v.g.vassilev]

@ellert ping...

[v.g.vassilev]

@uweigand, you are absolutely right. Iterating on that with the author we will drop this patch from our codebase too. Thanks for the help!!

cmake/config-ix.cmake

 elseif (LLVM_NATIVE_ARCH MATCHES "mips")
   set(LLVM_NATIVE_ARCH Mips)
 elseif (LLVM_NATIVE_ARCH MATCHES "xcore")
   set(LLVM_NATIVE_ARCH XCore)
 elseif (LLVM_NATIVE_ARCH MATCHES "msp430")
   set(LLVM_NATIVE_ARCH MSP430)
 elseif (LLVM_NATIVE_ARCH MATCHES "hexagon")
   set(LLVM_NATIVE_ARCH Hexagon)
-elseif (LLVM_NATIVE_ARCH MATCHES "s390x")
+elseif (LLVM_NATIVE_ARCH MATCHES "s390[x]")
   set(LLVM_NATIVE_ARCH SystemZ)
 elseif (LLVM_NATIVE_ARCH MATCHES "wasm32")
   set(LLVM_NATIVE_ARCH WebAssembly)
 elseif (LLVM_NATIVE_ARCH MATCHES "wasm64")
   set(LLVM_NATIVE_ARCH WebAssembly)
 else ()
   message(FATAL_ERROR "Unknown architecture ${LLVM_NATIVE_ARCH}")
 endif ()