deps: upgrade to libuv 1.38.0

Notable changes:

• uv_library_shutdown() has been added.
• uv_udp_init_ex() now accepts UV_UDP_RECVMMSG, although it is a no-op.
• Obsolete MAX_PATH restrictions have been removed on Windows, and Windows is now long path aware.
• Windows environment variables longer than 32,767 characters are now supported.
• Linux cpu_times are now reported as milliseconds to match other platforms.
• A memory leak resulting from uv_loop_init() failures has been fixed.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines