eliminate posix_spawn from the standard library

Today I found out that posix_spawn is trash. It's actually implemented
on top of fork/exec inside of libc (or libSystem in the case of macOS).

So, anything posix_spawn can do, we can do better. In particular, what
we can do better is handle spawning of child processes that are
potentially foreign binaries. If you try to spawn a wasm binary, for
example, posix spawn does the following:

 * Goes ahead and creates a child process.
 * The child process writes "foo.wasm: foo.wasm: cannot execute binary file"
   to stderr (yes, it prints the filename twice).
 * The child process then exits with code 126.

This behavior is indistinguishable from the binary being successfully
spawned, and then printing to stderr, and exiting with a failure -
something that is an extremely common occurrence.

Meanwhile, using the lower level fork/exec will simply return ENOEXEC
code from the execve syscall (which is mapped to zig error.InvalidExe).

The posix_spawn behavior means the zig build runner can't tell the
difference between a failure to run a foreign binary, and a binary that
did run, but failed in some other fashion. This is unacceptable, because
attempting to excecve is the proper way to support things like Rosetta.

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