linux: fix arm64 SYS__sysctl build breakage
linux: fix arm64 SYS__sysctl build breakage

The arm64 architecture never had a _sysctl system call and therefore
doesn't have a SYS__sysctl define either. Always return UV_ENOSYS.

Fixes: https://github.com/libuv/libuv/issues/2522
PR-URL: https://github.com/libuv/libuv/pull/2524
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso