Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1553

OpenBSD will remove its generic syscall(2) interface, so setting the ABSL_HAVE_SYSCALL_WRITE define will result in a linking failure soon.

Make direct use of the write(2) syscall instead. OpenBSD's libc does not do any buffering for write, so there is no change of behavior.

A [variant of this patch][1] has been in use since early this year in OpenBSD's ports. There's no need to set ABSL_LOW_LEVEL_WRITE_SUPPORTED since that's already done a few lines up.

[1]: https://github.com/openbsd/ports/commit/5f9e56cd982c1f1fa5af867e56355091e1817786
Merge 0dcc88a3cdab513c598587d85423135e85cec330 into b841db22

Merging this change closes #1553

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1553 from botovq:openbsd-syscall 0dcc88a3cdab513c598587d85423135e85cec330
PiperOrigin-RevId: 576552197
Change-Id: I13466703ddc9d50edf87da5d0c291aad642af49a