bug#64216: libgnu __spawni fail on Darwin

Due to the lack of attaching child process feature in LLDB, I cannot see what was happening after a fork() call, but one thing is clear that the alternative `posix_spawn` provided by lib/spawni.c was not quite working as expected on Darwin systems (might only happens under arm64). 

https://github.com/NanoComp/meep/issues/2495 <https://github.com/NanoComp/meep/issues/2495>

Same bug mentioned in this GitHub issue, and I have actually attempted to undef REPLACE_POSIX_SPAWN to use system one, but it was still corrupted by malformed file actions which returning 22 (EINVAL) and set errno to ENOENT that may complaining about dup2-ed non-standard stdio fds.

One thing is for sure, when REPLACE_POSIX_SPAWN is defined, posix_spawn calls will point to __spawni function as implementation. This function fails before actually calling `execve` (Or I didn't successfully caught that in lldb)

This was my test result on macOS 11.2 arm64

/Applications/Xcode.app/Contents/Developer/usr/bin/make  check-TESTS
test-system-cmds: system* exit status was 127 rather than 42
FAIL: test-system-cmds
PASS: test-bad-identifiers
PASS: test-require-extension
PASS: test-guile-snarf
PASS: test-import-order
PASS: test-command-line-encoding
PASS: test-command-line-encoding2
PASS: test-language
error: interrupted by the user
PASS: test-guild-compile
wrote `/Users/torrekie/proj/guile-3.0.9/cache/guile/ccache/3.0-LE-8-4.6/Users/torrekie/proj/guile-3.0.9/test-suite/standalone/test-signal-fork.go'
parent: 53087
....child: ..53133
.............................................
completed
PASS: test-signal-fork
PASS: test-num2integral
PASS: test-round
PASS: test-asmobs
PASS: test-ffi
PASS: test-foreign-object-scm
PASS: test-foreign-object-c
PASS: test-list
PASS: test-unwind
PASS: test-conversion
PASS: test-loose-ends
PASS: test-fast-slot-ref
PASS: test-mb-regexp
PASS: test-use-srfi
PASS: test-scm-c-read
PASS: test-scm-take-locale-symbol
PASS: test-scm-take-u8vector
PASS: test-scm-to-latin1-string
PASS: test-scm-values
PASS: test-scm-c-bind-keyword-arguments
PASS: test-srfi-4
PASS: test-extensions
PASS: test-with-guile-module
PASS: test-scm-with-guile
PASS: test-scm-spawn-thread
PASS: test-pthread-create
SKIP: test-pthread-create-secondary
PASS: test-smob-mark
PASS: test-smob-mark-race
wrote `/Users/torrekie/proj/guile-3.0.9/cache/guile/ccache/3.0-LE-8-4.6/Users/torrekie/proj/guile-3.0.9/test-suite/standalone/test-stack-overflow.go'
SKIP: test-stack-overflow
wrote `/Users/torrekie/proj/guile-3.0.9/cache/guile/ccache/3.0-LE-8-4.6/Users/torrekie/proj/guile-3.0.9/test-suite/standalone/test-out-of-memory.go'
SKIP: test-out-of-memory

;;; (child-exception ("scm_fdes_to_port" "~A" ("Bad file descriptor") (9)))

;;; (child-status 256)
PASS: test-close-on-exec
==================================
1 of 38 tests failed
(3 tests were not run)
Please report to [email protected]
==================================
make[5]: *** [check-TESTS] Error 1
make[4]: *** [check-am] Error 2
make[3]: *** [check] Error 2
make[2]: *** [check-recursive] Error 1
make[1]: *** [check-recursive] Error 1
make: *** [check] Error 2