Reimplement addbuilddir() in C inside getpath.c, so as to execute it

at interpreter startup before importing any non-builtin modules.
Should fix #9589.

Lib/site.py

@@ -107,18 +107,6 @@ def removeduppaths():
     sys.path[:] = L
     return known_paths
 
-# XXX This should not be part of site.py, since it is needed even when
-# using the -S option for Python.  See http://www.python.org/sf/586680
-def addbuilddir():
-    """Append ./build/lib.<platform> in case we're running in the build dir
-    (especially for Guido :-)"""
-    from sysconfig import get_platform
-    s = "build/lib.%s-%.3s" % (get_platform(), sys.version)
-    if hasattr(sys, 'gettotalrefcount'):
-        s += '-pydebug'
-    s = os.path.join(os.path.dirname(sys.path.pop()), s)
-    sys.path.append(s)
-
 
 def _init_pathinfo():
     """Return a set containing all existing directory entries from sys.path"""
@@ -529,9 +517,6 @@ def main():
 
     abs_paths()
     known_paths = removeduppaths()
-    if (os.name == "posix" and sys.path and
-        os.path.basename(sys.path[-1]) == "Modules"):
-        addbuilddir()
     if ENABLE_USER_SITE is None:
         ENABLE_USER_SITE = check_enableusersite()
     known_paths = addusersitepackages(known_paths)