v9.3.1 released

Fun 9.3.1

The interpreter cross-compiles for macOS on a Linux host.

Highlights since v9.3.0:

- bootstrap-darwin-cross.sh builds a full FPC 3.2.2 darwin cross toolchain
  (ppcrossx64/ppcrossa64 plus the RTL, the rtl-objpas/hash units and PCRE) and
  fetches Zig; make-darwin.sh compiles funcmd.dpr into fun/funcmd-darwin-<cpu>.

- Mach-O assembly and linking go through the darwin/clang and darwin/ld shims
  over Zig's bundled clang/lld, so no macOS SDK is required.

- The engine's {$IfDef Linux} guards that really meant "not Windows" now use
  FPC's built-in UNIX symbol: core/{base,fun,io}, parse/parse, utils/utils, the
  lib helpers, regex/pcre/* and funcmd.dpr. host.arg() reports os macos, Darwin
  gets clock_gettime/gettimeofday and cdecl PCRE externals.

- Linux is unchanged: the regression suite stays at 91/91. The Fun-level -lnx
  libraries and the getapi FFI backend are not ported yet.