Added e2k support (MCST Elbrus 2000) (#5222)

- Add e2k target to config.guess
- Update config-ix.cmake to support e2k

E2K - this is VLIW/EPIC architecture, like Intel Itanium (IA-64)
architecture.

Ref:
1) https://en.wikipedia.org/wiki/Elbrus_2000
2)
https://lists.gnu.org/archive/html/config-patches/2015-03/msg00000.html

autoconf/config.guess

@@ -909,6 +909,9 @@ EOF
     crisv32:Linux:*:*)
 	echo crisv32-axis-linux-gnu
 	exit ;;
+    e2k:Linux:*:*)
+	echo ${UNAME_MACHINE}-unknown-linux-gnu
+	exit ;;
     frv:Linux:*:*)
 	echo frv-unknown-linux-gnu
 	exit ;;

cmake/config-ix.cmake

@@ -367,6 +367,8 @@ elseif (LLVM_NATIVE_ARCH MATCHES "wasm64")
   set(LLVM_NATIVE_ARCH WebAssembly)
 elseif (LLVM_NATIVE_ARCH MATCHES "riscv64")
   set(LLVM_NATIVE_ARCH RISCV)
+elseif (LLVM_NATIVE_ARCH MATCHES "e2k")
+  set(LLVM_NATIVE_ARCH E2K)
 else ()
   message(FATAL_ERROR "Unknown architecture ${LLVM_NATIVE_ARCH}")
 endif ()