[CMake][AIX] quote the string AIX if conditions

This is a follow on to #154537, which quoted the CMAKE_SYSTEM_NAME to avoid expanding it again when that CMAKE_SYSTEM_NAME expands to AIX.

But by the same logic, we also need to quote the plain string AIX as well.

Author: daltenty

llvm/cmake/modules/GetHostTriple.cmake

@@ -34,7 +34,7 @@ function( get_host_triple var )
     endif()
   elseif( CMAKE_SYSTEM_NAME MATCHES "OS390" )
     set( value "s390x-ibm-zos" )
-  elseif( CMAKE_SYSTEM_NAME STREQUAL AIX )
+  elseif( CMAKE_SYSTEM_NAME STREQUAL "AIX" )
     # We defer to dynamic detection of the host AIX version.
     if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "powerpc64-ibm-aix" )

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1330,7 +1330,7 @@ endif()
 # linking (due to incompatibility). With MSVC, note that the plugin has to
 # explicitly link against (exactly one) tool so we can't unilaterally turn on
 # LLVM_ENABLE_PLUGINS when it's enabled.
-if("${CMAKE_SYSTEM_NAME}" MATCHES AIX)
+if("${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
   set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS_OPTION OFF)
 else()
   set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS_OPTION ON)

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

@@ -99,7 +99,7 @@ function(llvm_ExternalProject_Add name source_dir)
       list(APPEND ARG_TOOLCHAIN_TOOLS flang)
     endif ()
     # AIX 64-bit XCOFF and big AR format is not yet supported in some of these tools.
-    if(NOT _cmake_system_name STREQUAL AIX)
+    if(NOT _cmake_system_name STREQUAL "AIX")
       list(APPEND ARG_TOOLCHAIN_TOOLS lld llvm-ar llvm-ranlib llvm-nm llvm-objdump)
       if(_cmake_system_name STREQUAL Darwin)
         list(APPEND ARG_TOOLCHAIN_TOOLS llvm-libtool-darwin llvm-lipo)

llvm/tools/lto/CMakeLists.txt

@@ -25,7 +25,7 @@ if(LLVM_ENABLE_PIC)
 
   set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_CURRENT_SOURCE_DIR}/lto.exports)
 
-  if(CMAKE_SYSTEM_NAME STREQUAL AIX)
+  if(CMAKE_SYSTEM_NAME STREQUAL "AIX")
       set(LTO_LIBRARY_TYPE MODULE)
       set(LTO_LIBRARY_NAME libLTO)
     else()