Hyperref with align inside subequations

You can move the actual mark down by 2 baseline skips (1 to counter the default raise and another to bring it to the first equation) and then force a new target using \phantomsection:

\documentclass{article}

\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{cleveref}

\begin{document}

Some text before the subequations.
\begin{subequations}
  \raisebox{-2\baselineskip}[0pt][0pt]{%
    \phantomsection % Set new hyper target
    \label{eq:arithmetic} % Mark label
  }
  \begin{align}
    a + b &= c \\
    a + b &= c \\
    a + b &= c \\
    a + b &= c
  \end{align}
\end{subequations}
The reference to the main equation with \cref{eq:arithmetic} brings me to a line before the set of subequations.

\end{document}

We use \raisebox with a 0pt resulting depth and 0pt height (similar to \smash).

New answer

The problem with the subequations environment is that it is not a math environment. The subequations environment merely changes the behavior of the counter of numbered equations.

As a result, when a hyperlink anchor is created for the main subequations, its position is actually on the baseline of the previous last line of text. To see this, uncomment the line \setlength\HyperRaiseLinkLength{0pt} in the solution below.

So, if there is a page break before the subequations environment, the hyperlink anchor has nowhere to go but the upper-left corner of the text body in the new page. There isn’t much hyperref can do in this case. We could rewrite the text:

1. Make it longer so one or two lines follow onto the next page;
2. Make it shorter and use \allowdisplaybreaks so the first line of subequations fits in the previous page.

In any case, avoid starting a new page with equations.

With the unpleasant “new page” situation out of the way, I provide a new implementation of shifting the hyperlink anchor. Now, the hyperlinks for eqs. (1) and (1a) point to the exact same spot!

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{cleveref}

% Let's patch \HyOrg@subequations from hyperref
\usepackage{etoolbox}
\makeatletter
% Change \HyperRaiseLinkLength for the main anchor in subequations
\preto\HyOrg@subequations{%
  \renewcommand*\HyperRaiseLinkHook{%
    % Drop the main anchor from the baseline of previous text by \abovedisplayskip
    \setlength\HyperRaiseLinkLength{-\abovedisplayskip}%
    % Then, raise the main anchor by 3pt
    % (The 3pt is to cancel the -\lineskip from \displ@y, see amsldoc.tex)
    \addtolength\HyperRaiseLinkLength{3pt}%
    % To see the default position of the main anchor, uncomment the following line:
    %\setlength\HyperRaiseLinkLength{0pt}%
  }%
}
% Change nothing for the other anchors in subequations
\appto\HyOrg@subequations{%
  \let\HyperRaiseLinkHook\@empty
}
\makeatother

\begin{document}
    Some text before the subequations.
    \begin{subequations}\label{eq:arithmetic}
        \begin{align}
            a + b &= c \label{eq:arithmetic-a} \\
            a + b &= c \label{eq:arithmetic-b} \\
            a + b &= c \nonumber \\
            a + b &= c \label{eq:arithmetic-c}
        \end{align}
    \end{subequations}
    Testing:
    \Cref{eq:arithmetic,eq:arithmetic-a} bring me to the \emph{exact} same place,
    while \cref{eq:arithmetic-b,eq:arithmetic-c} bring me to the correct subequations.
    \begin{equation}
        e^{i\pi} = -1 \label{eq:euler}
    \end{equation}
    Testing:
    \Cref{eq:euler} bring me to Euler's identity.
\end{document}

---

Old answer

An extremely dirty hack:

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{cleveref}

% Since you have loaded cleveref, we can patch \cref@old@subequations directly
\usepackage{etoolbox}
\makeatletter
% Vertical space No. 1:
\preto\cref@old@subequations{\vskip\abovedisplayskip\nobreak}
% Vertical space No. 2:
\appto\cref@old@subequations{\vskip-\abovedisplayskip\vskip-\baselineskip\nobreak}
\makeatother

\begin{document}
    Some text before the subequations.
    \begin{subequations}\label{eq:arithmetic}
        \begin{align}
            a + b &= c \\
            a + b &= c \\
            a + b &= c \\
            a + b &= c
        \end{align}
    \end{subequations}
    The reference to the main equation with \cref{eq:arithmetic} brings me to a line before the set of subequations.
\end{document}