Reland "[CMake] Bumps minimum version to 3.20.0."

The owner of the last two failing buildbots updated CMake.

This reverts commit e8e8707b4aa6e4cc04c0cffb2de01d2de71165fc.

GitOrigin-RevId: 65429b9af6a2c99d340ab2dcddd41dab201f399c
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6f12a16..bd49dfb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,7 +2,7 @@
 # Setup Project
 #===============================================================================
 
-cmake_minimum_required(VERSION 3.13.4)
+cmake_minimum_required(VERSION 3.20.0)
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
 
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 2eabbe0..ef34174 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -30,8 +30,7 @@
 # CMake doesn't work correctly with assembly on AIX. Workaround by compiling
 # as C files as well.
 if((APPLE AND CMAKE_VERSION VERSION_LESS 3.19) OR
-   (MINGW AND CMAKE_VERSION VERSION_LESS 3.17) OR
-   (${CMAKE_SYSTEM_NAME} MATCHES "AIX"))
+   (MINGW AND CMAKE_VERSION VERSION_LESS 3.17))
   set_source_files_properties(${LIBUNWIND_ASM_SOURCES} PROPERTIES LANGUAGE C)
 endif()