BUILD: import/mt_list: support building with TCC

BUILD: import/mt_list: support building with TCC

TCC is often convenient to quickly test builds, run CI tests etc. It has
limited thread support (e.g. no thread-local stuff) but that is often
sufficient for testing. TCC lacks __atomic_exchange_n() but has the
exactly equivalent __atomic_exchange(), and doesn't have any barrier.
For this reason we force the atomic_exchange to use the stricter SEQ_CST
mem ordering that allows to ignore the barrier.

[wt: that's upstream commit ca8b865 ("BUILD: support building with TCC")]