Add SIMDE support for portable SIMD on non-x86 platforms

[wszqkzqk]

This PR adds optional support for SIMDE (SIMD Everywhere). This allows the existing SSE-optimized codebase to compile and run with hardware acceleration on non-x86 architectures (such as ARM64/NEON, RISC-V, POWER, and WASM) without rewriting the core algorithm.

Features

• The default behavior is unchanged. Native reference xmmintrin.h usage is preserved unless KISSFFT_USE_SIMDE is explicitly enabled.
• The x86-specific _mm_malloc is replaced with portable aligned allocation strategies (C11 aligned_alloc, POSIX posix_memalign, or MSVC _aligned_malloc) only when building in SIMDE mode.
• Follows SIMDE best practices by avoiding SIMDE_ENABLE_NATIVE_ALIASES. Instead, it uses simde_ prefixed intrinsics to prevent conflicts with system headers.

Build

CMake:

cmake -DKISSFFT_DATATYPE=simd -DKISSFFT_USE_SIMDE=ON ..

Makefile:

make KISSFFT_DATATYPE=simd KISSFFT_USE_SIMDE=1

Testing

I've verified that standard x86 builds (KISSFFT_DATATYPE=simd) compile with -msse and behave identically to master and SIMDE builds compile successfully and pass test/tsimd.

I've also verified it on LoongArch64. Tests were performed on LoongArch64 (Arch Linux for Loong64, Loongson 3C5000L).

Benchmark Results (nfft=1800)

• Scalar (float): 1.820s
• SIMD (SIMDe): 0.510s

~3.57x faster on Loongson 3C5000L.

With ctest:

    Start 1: bm_kiss
1/9 Test #1: bm_kiss ..........................   Passed    0.06 sec
    Start 2: bm_fftw
2/9 Test #2: bm_fftw ..........................   Passed    0.00 sec
    Start 3: st
3/9 Test #3: st ...............................   Passed    0.01 sec
    Start 4: tkfc
4/9 Test #4: tkfc .............................   Passed    0.00 sec
    Start 5: ffr
5/9 Test #5: ffr ..............................   Passed    0.00 sec
    Start 6: tr
6/9 Test #6: tr ...............................   Passed    0.14 sec
    Start 7: testcpp
7/9 Test #7: testcpp ..........................   Passed  301.38 sec
    Start 8: tsimd
8/9 Test #8: tsimd ............................   Passed    0.02 sec
    Start 9: testkiss.py
9/9 Test #9: testkiss.py ......................   Passed    0.21 sec

100% tests passed, 0 tests failed out of 9

Total Test time (real) = 301.84 sec

With Makefile:

since SIMD implementation does 4 ffts at a time, numffts is being reduced to 2500
KISS    nfft=1800,      numffts=2500
COMMAND         MAJFLT MINFLT   RSS   DRS PAGEIN    SZ  TRS    VSZ
bm-kiss-simd         0    159  2240  3672      0   230    7   3680
        cputime=0.510
Datatype not available in FFTW
testkiss.py does not yet test simdg++ -o testcpp-simd -O3 -ffast-math -fomit-frame-pointer  -I.. -W -Wall -march=native -mtune=native testcpp.cc -L.. -lkissfft-simd -lm
LD_LIBRARY_PATH=":.." ./testcpp-simd
type:f nfft:32 RMSE:9.35194e-08  MSPS:89.9729
type:d nfft:32 RMSE:1.26246e-16  MSPS:91.6864
type:e nfft:32 RMSE:3.8172e-33   MSPS:0.900086
type:f nfft:1024 RMSE:1.81694e-07        MSPS:60.6544
type:d nfft:1024 RMSE:3.30505e-16        MSPS:60.6483
type:e nfft:1024 RMSE:1.38033e-31        MSPS:0.386512
type:f nfft:840 RMSE:5.01374e-07         MSPS:33.8333
type:d nfft:840 RMSE:3.22384e-16         MSPS:30.0022
type:e nfft:840 RMSE:1.09615e-31         MSPS:0.201911