wasm: guard remaining KERNEL assignments so KERNEL.$(TARGET_CORE) overrides survive

[MMesch]

Fixes #5993. Base kernel/wasm/KERNEL is included after KERNEL.$(TARGET_CORE) (kernel/Makefile:119-121), so unguarded = assignments in the base file clobber per-target overrides. #5984 (AXPY), #5983 (GEMM/TRMM), and #5990 (GEMV) fixed this family-by-family; this PR wraps every remaining assignment in ifndef so future WASM sub-targets can override any kernel without a matching guard edit here.

On develop, the still-unguarded families this PR moves are SDOT / DDOT / SROT / DROT. SIMD-opcode counts in libopenblas.so for these kernels (measured against the v0.3.34 tag, since that predates the prior guard PRs):

kernel	simd_before	simd_after
sdot_k	0	27
ddot_k	0	25
srot_k	12	74
drot_k	12	66

Downstream (emscripten-forge/recipes#6343): 1.5-2x speedup on numpy.correlate under Node/Emscripten (TARGET=WASM128_GENERIC, USE_THREAD=0).

Tested with utest and CBLAS ctest L1/L2/L3 under Node.

[martin-frbg]

Thank you