[3.6] bpo-35257: Avoid leaking LTO linker flags into distutils (GH-10900) (GH-11265)

[3.6] bpo-35257: Avoid leaking LTO linker flags into distutils (GH-10900) (GH-11265)

When compiling 3rd party C extensions, the linker flags used by the
compiler for the interpreter and the stdlib modules, will get
leaked into distutils. In order to avoid that, the PY_CORE_LDFLAGS
and PY_LDFLAGS_NODIST are introduced to keep those flags separated.

(cherry picked from commit cf10a750f4b50b6775719cfb17bee00bc3a9c60b)