Build: Use wrapper script for gas-preprocessor.pl

The previous hack (adding ${CMAKE_ASM_COMPILER} to CMAKE_ASM_FLAGS)
didn't work in all cases, because more recent versions of CMake place
the includes ahead of the flags (which meant that the real assembler
wasn't the first argument to gas-preprocessor.pl.)

simd/CMakeLists.txt

@@ -228,9 +228,8 @@ if(NOT RESULT EQUAL 0)
     return()
   else()
     message(STATUS "Using gas-preprocessor.pl")
-    set(CMAKE_REAL_ASM_COMPILER ${CMAKE_ASM_COMPILER})
-    set(CMAKE_ASM_COMPILER gas-preprocessor.pl)
-    set(CMAKE_ASM_FLAGS "${CMAKE_REAL_ASM_COMPILER} ${CMAKE_ASM_FLAGS}")
+    configure_file(gas-preprocessor.in gas-preprocessor @ONLY)
+    set(CMAKE_ASM_COMPILER ${CMAKE_CURRENT_BINARY_DIR}/gas-preprocessor)
   endif()
 else()
   message(STATUS "GAS is working properly")

simd/gas-preprocessor.in

@@ -0,0 +1 @@
+gas-preprocessor.pl @CMAKE_ASM_COMPILER@ ${1+"$@"}