[CMake] Bumps minimum version to 3.20.0.

This partly undoes D137724.

This change has been discussed on discourse
https://discourse.llvm.org/t/rfc-upgrading-llvms-minimum-required-cmake-version/66193

Note this does not remove work-arounds for older CMake versions, that
will be done in followup patches.

Reviewed By: mehdi_amini, MaskRay, ChuanqiXu, to268, thieta, tschuett, phosek, #libunwind, #libc_vendors, #libc, #libc_abi, sivachandra, philnik, zibi

Differential Revision: https://reviews.llvm.org/D144509

GitOrigin-RevId: 44c6b905f8527635e49bb3ea97dea315f92d38ec
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f82fa0a..bc667c8 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 df32e53..275cabc 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -35,8 +35,7 @@
 # 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()