[stable] Slightly relax gammafunction unittests to pass with quadruple real

[kinke]

This fixes the unittest failures on my Raspberry Pi 5 (based on Debian 13), with LDC.

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[kinke]

Unsure about this one. Edit: The beta(…) result is a NaN apparently. Edit2: Whereas it's a negative NaN with 80-bit real.

[kinke]

Changed to version(AArch64), as beta() yields a positive NaN for macOS arm64 CI too, with 64-bit real. Note that this AArch64/ARM(?) behavior isn't affected by setting the non-fast FPU mode in https://github.com/dlang/dmd/blob/c8e2ae291414f839de5a7635da09a03c63fed2f6/druntime/src/test_runner.d#L114-L152.