compiler_rt: declutter int.zig, add mulXi3 tests (#14623)

- Combine mulXi3 routines for follow-up cleanup.
- DRY up Dwords and Twords
- rename both to HalveInt and use instance
  * Justification: Not all processors have word size 32 bit.
* remove test file from CMakeLists
* DRY things.

CMakeLists.txt

@@ -434,13 +434,12 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/log10.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/log2.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/modti3.zig"
+    "${CMAKE_SOURCE_DIR}/lib/compiler_rt/mulXi3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/muldf3.zig"
-    "${CMAKE_SOURCE_DIR}/lib/compiler_rt/muldi3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/mulf3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/mulo.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/mulsf3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/multf3.zig"
-    "${CMAKE_SOURCE_DIR}/lib/compiler_rt/multi3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/mulxf3.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/negXi2.zig"
     "${CMAKE_SOURCE_DIR}/lib/compiler_rt/negv.zig"
@@ -613,7 +612,6 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/src/link/tapi.zig"
     "${CMAKE_SOURCE_DIR}/src/link/tapi/Tokenizer.zig"
     "${CMAKE_SOURCE_DIR}/src/link/tapi/parse.zig"
-    "${CMAKE_SOURCE_DIR}/src/link/tapi/parse/test.zig"
     "${CMAKE_SOURCE_DIR}/src/link/tapi/yaml.zig"
     "${CMAKE_SOURCE_DIR}/src/main.zig"
     "${CMAKE_SOURCE_DIR}/src/mingw.zig"
@@ -753,7 +751,7 @@ set(BUILD_ZIG2_ARGS
   --deps build_options
   -target "${HOST_TARGET_TRIPLE}"
 )
- 
+
 add_custom_command(
   OUTPUT "${ZIG2_C_SOURCE}"
   COMMAND zig1 ${BUILD_ZIG2_ARGS}
@@ -771,7 +769,7 @@ set(BUILD_COMPILER_RT_ARGS
   --deps build_options
   -target "${HOST_TARGET_TRIPLE}"
 )
- 
+
 add_custom_command(
   OUTPUT "${ZIG_COMPILER_RT_C_SOURCE}"
   COMMAND zig1 ${BUILD_COMPILER_RT_ARGS}