python3: Enable compile-time optimizations

python3: Enable compile-time optimizations

This adds the --enable-optimizations configure option (for
profile-guided optimization) for both host and target Python, and the
--with-lto configure option (for link-time optimization) for target
Python (for non-MIPS platforms).

Currently, compiling Python with LTO leads to link errors on mips and
mipsel. (Compiling with LTO appears to succeed on mips64 but there is
only one mips64 target available for convenient testing.)

This also cleans up the host and target configure options:

* Sort options/variables
  - Alphabetically
  - Flags/options before child-process environment variables
  - Group options by type (enable/disable/with/without)
  - Static options/variables before conditional ones

* Remove the prefix/dir options, as they are the same as the defaults
  set by the build system

* Remove --with-threads, as it is no longer a valid option (threads are
  always enabled)

Signed-off-by: Jeffery To <jeffery.to@gmail.com>