Add m68k to SwiftSetIfArchBitness.cmake (#70668)

`m68k` is a 32-bit architecture and should be defined as such in that file.

Author: MaxDesiatov

cmake/modules/SwiftSetIfArchBitness.cmake

@@ -18,6 +18,7 @@ function(set_if_arch_bitness var_name)
      "${SIA_ARCH}" STREQUAL "armv7m" OR
      "${SIA_ARCH}" STREQUAL "armv7em" OR
      "${SIA_ARCH}" STREQUAL "armv7s" OR
+     "${SIA_ARCH}" STREQUAL "m68k" OR
      "${SIA_ARCH}" STREQUAL "riscv32" OR
      "${SIA_ARCH}" STREQUAL "wasm32" OR
      "${SIA_ARCH}" STREQUAL "powerpc")