unistd: remove fork/exec on fuchsia

[xtqqczze]

On Fuchsia, these functions are intentionally unimplemented and return ENOSYS.

References:

• https://fuchsia.dev/fuchsia-src/development/languages/c-cpp/libc#posix
• https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/third_party/ulib/musl/src/process/fork.c
• https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/third_party/ulib/musl/src/process/execv.c
• https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/third_party/ulib/musl/src/process/execve.c

[xtqqczze]

Hmm, it’s surprising that the test_wait_signal test would have previously passed if fork always returned an Err on Fuchsia:

error[E0425]: cannot find function `fork` in this scope
  --> test/sys/test_wait.rs:15:20
   |
15 |     match unsafe { fork() }.expect("Error: Fork Failed") {
   |                    ^^^^ not found in this scope
   |

[stepancheg]

Even if functions are unimplemented, it is often convenient to have them, so you don't have to turn your project into endless cfg-guards.

In fact, this is likely the reason why these function are declared in fuchsia, to make writing cross-platform code easier.

(I have very little to do with this project, so this is my 2 cents.)