Ignoring 'target specific option mismatch' on inline

[J.W. Jagersma]

Consider the following (incomplete) code:

    enum simd { mmx = 1, sse = 2 /* , ... */ };

    template<simd flags>
    void simd_func()
    {
        if constexpr (flags & simd::mmx)
        {
            // MMX code here
        }
        else if constexpr (flags & simd::sse)
        {
            // SSE code here
        }
        // etc ...
    }

The idea is to instantiate function templates with optimized SIMD routines
multiple times, then I can compile all my code with -march=i386, and select the
best implementation at runtime.

However, after already spending a lot of time restructuring my code around this
idea, I discover that gcc refuses to compile this without having the
corresponding target options enabled:

    error: inlining failed in call to 'always_inline' '__m64 _mm_unpacklo_pi8(__m64, __m64)': target specific option mismatch

That makes no sense to me.  I want gcc to emit those SIMD instructions verbatim
in my code, regardless of what compiler options the user specified.  What then
is the point of feature-test macros (__SSE__, etc)?  I don't get it.

Is there any option or attribute I can use to make gcc ignore these target
option mismatches when inlining?  Or any other way to make this work?