[3.6] bpo-31354: Let configure --with-lto work on all builds (GH-10261)

[3.6] bpo-31354: Let configure --with-lto work on all builds (GH-10261)

Allow configure --with-lto to apply to all builds, not just profile-opt builds.

Whether this is actually useful or not must be determined by the person
building CPython using their own toolchain.

My own quick test on x86_64 Debian 9 (gcc 6.3, binutils 2.28) seemed
to suggest that it wasn't, but I expect better toolchains can or will exist
at some point.  The point is to allow it at all.