oneTBB API is changing rapidly, so even if a oneTBB is installed
into a system already, it is likely that that is not a compatible
version with mold. This makes program building unnecessarily hard.
This change intends to solve the problem once and for all by
including oneTBB as a subdirectory and static-link against it at
build-time.
oneTBB is released under the Apache License 2.0, which is compatible
with AGPL. Therefore, we can still distribute the binary of mold
under AGPL.
If you do not want to static-link oneTBB, pass `SYSTEM_TBB=1`
to `make`.
This change also ports mold from oneTBB v2020.3 to v2021.3.0.
Fixes https://github.com/rui314/mold/issues/66