add min_pos_value constant for floats

Follow-up on issue #13297 and PR #13710.  Instead of following the (confusing) C/C++ approach
of using `MIN_VALUE` for the smallest *positive* number, we introduce `MIN_POS_VALUE` (and
in the Float trait, `min_pos_value`) to represent this number.

This patch also removes a few remaining redundantly-defined constants that were missed last
time around.
test: Enable extern-fn-reachable test

It didn't work because it tried to call itself but symbols are not
exported as default in executables.

Note that `fun5` is not internal anymore since it is in library.
test: Remove/update some old ignored tests
auto merge of #13741 : klutzy/rust/test-reachable, r=alexcrichton

It didn't work because it tried to call itself but symbols are not
exported as default in executables.

Note that `fun5` is not internal anymore since it is in library.

Second commit removes/updates some old tests.
auto merge of #13735 : aturon/rust/float-consts-take-2, r=brson

Follow-up on issue #13297 and PR #13710.  Instead of following the (confusing) C/C++ approach
of using `MIN_VALUE` for the smallest *positive* number, we introduce `MIN_POS_VALUE` (and
in the Float trait, `min_pos_value`) to represent this number.

This patch also removes a few remaining redundantly-defined constants that were missed last
time around.