Simplify binary paths

These old names were for gyp, and are not needed for gn, so we can
instead use the same path names Qt would use.

Change-Id: I423c2a04df6740ebba84f8c670490dbcf59c3ca7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

mkspecs/features/functions.prf

 defineReplace(getConfigDir) {
-  win32:contains(QMAKE_TARGET.arch, x86_64) {
-      CONFIG(release, debug|release):return("Release_x64")
-      return("Debug_x64")
-  }
-
-  CONFIG(release, debug|release):return("Release")
-  return("Debug")
+  CONFIG(release, debug|release):return("release")
+  return("debug")
 }
 
 defineReplace(getChromiumSrcDir) {