Revert "Interpret x86_64 as x86-64 for architecture"

This reverts commit 8623492150.

Let's restrict ARCH to values accepted by -march to keep things clear
and consistent. ARCH is -march, with only one exception: a value of
"default" indicates to not pass -march at all.

CMakeLists.txt

@@ -312,11 +312,7 @@ else()
   if(ARCH STREQUAL "default")
     set(ARCH_FLAG "")
   else()
-    if(ARCH STREQUAL "x86_64")
-      set(ARCH_FLAG "-march=x86-64")
-    else()
-       set(ARCH_FLAG "-march=${ARCH}")
-    endif()
+    set(ARCH_FLAG "-march=${ARCH}")
   endif()
   set(WARNINGS "-Wall -Wextra -Wpointer-arith -Wundef -Wvla -Wwrite-strings -Wno-error=extra -Wno-error=deprecated-declarations -Wno-unused-parameter -Wno-unused-variable -Wno-error=unused-variable -Wno-error=undef -Wno-error=uninitialized")
   if(NOT MINGW)