system.h: drop compilers lacking 64-bit integer type (Windows/MS-DOS)

system.h: drop compilers lacking 64-bit integer type (Windows/MS-DOS)

- DJGPP 1.x (including `__GO32__`) (MS-DOS)
  DJGPP 2.x support remains unchanged.
- Salford C (Windows)
- Turbo C (Windows 16-bit)
- Borland C++ < 5.2 (Windows 16-bit?)
- Pelles C < 2.8 (Windows)

These targets mapped `curl_off_t` to `long`. On Windows and MS-DOS
`long` is always 32-bit.

curl requires C compilers supporting 64-bit `curl_off_t` type since
835682661c7a36b0e860c978204e8cd9b033be45 #10597 (v8.0.0).

Also: drop remaining `__GO32__` and Salford C guards.

Closes #15957