fix: performance problem in crashReporter.start() on macOS - #34638

This change reduces the duration of crashReporter.start() on Intel macOS
from 622 milliseconds to 257 milliseconds!

Backports https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3641386

  posix: Replace DoubleForkAndExec() with ForkAndSpawn()

  The DoubleForkAndExec() function was taking over 622 milliseconds to run
  on macOS 11 (BigSur) on Intel i5-1038NG7. I did some debugging by adding
  some custom traces and found that the fork() syscall is the bottleneck
  here, i.e., the first fork() takes around 359 milliseconds and the
  nested fork() takes around 263 milliseconds. Replacing the nested fork()
  and exec() with posix_spawn() reduces the time consumption to 257
  milliseconds!

  See libuv/libuv#3064 to know why fork() is so
  slow on macOS and why posix_spawn() is a better replacement.

  Another point to note is that even base::LaunchProcess() from Chromium
  calls posix_spawnp() on macOS -
  https://source.chromium.org/chromium/chromium/src/+/8f8d82dea0fa8f11f57c74dbb65126f8daba58f7:base/process/launch_mac.cc;l=295-296

  Change-Id: I25c6ee9629a1ae5d0c32b361b56a1ce0b4b0fd26
  Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3641386
  Reviewed-by: Mark Mentovai <mark@chromium.org>
  Commit-Queue: Mark Mentovai <mark@chromium.org>

Fixes: #34321
Signed-off-by: Darshan Sen <raisinten@gmail.com>