unix: fix UV_FS_O_DIRECT definition on Linux
unix: fix UV_FS_O_DIRECT definition on Linux

According to http://man7.org/linux/man-pages/man2/open.2.html, the
O_DIRECT flag is Linux-specific. One must define _GNU_SOURCE to obtain
the correct definition.

However, defining _GNU_SOURCE in the headers is unhygienic and affects
the libuv consumer.

On Linux, we now define UV_FS_O_DIRECT explicitly according to
architecture, only falling back to O_DIRECT if defined, or 0 if not.

Fixes: https://github.com/libuv/libuv/issues/2420
PR-URL: https://github.com/libuv/libuv/pull/2441
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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