🚀 go-pugleaf

Lots of uC's have both async and sync serial I/O on-chip.  The Z180 from
Zilog (second-sourced by Hitachi) is a good example.  That particular
chip isn't really a microcontroller, because it has no RAM or ROM on-chip.
Software tools are widely available, because it is code-compatible with the
Z80.

If we're talking about a one-of-a-kind application, NEC makes the 78213,
which is a Z80-like microcontroller with on-chip sync and async serial
I/O, in addition to a lot of other stuff like A/D.  It's available ROMless,
and with UV EPROM.  This might be a good choice for a one-of-a-kind system
because the evaluation board is reasonably priced.  It's $295, and that includes
an assembler and linker for the IBM PC.

The Motorola 68HC11 also has both sync and async serial I/O.  I think their 
evaluation board is even cheaper, like around $100.  If I remember correctly,
they've got a BBS from which you can download a C compiler.