Add (e)Z80 backend

[myclevorname]

The Zilog eZ80 is a backwards-compatible successor to the Zilog Z80.

The Z80 has had quite a few C compilers, like SDCC, z88dk, and many proprietary compilers that are no longer maintained. The eZ80, however, is in a tighter situation than the 6502 because the only two C compilers available (to my knowledge) are Zilog Developer Studio (proprietary, generates poor code, only supports C90 and C++-style comments) and @jacobly's fork of the LLVM toolchain (dormant due to his contributions to Zig).

Drawbacks

• The Z80 isn't very popular today, and the eZ80 less so. The Z80 isn't even sold anymore!

  • Some old architectures, like the MOS 6502 and Motorola 68000, are still popular enough to have some work put into them, whether upstream or downstream.

• (e)Z80 programs are sensitive to code size, so adding optimizations may be the difference between a program fitting on the device and not fitting at all.

Prerequisites

• Missing c_float and c_double types #3999
  • Zilog's eZ80 C ABI specifies that floats and doubles are 32 bits wide, and the CE toolchain (linked below) defined long double as 64 bits wide.
• TODO: find more

Resources

• Zilog eZ80 C Compiler manual
• Zilog Developer Studio II Calling Convention
• Third-party TI-84 Plus CE toolchain
• z88dk
• SDCC
• eZ80 assembly manual
• Z80 assembly manual (contains mistakes)