Fix typo ${tolchain} in build_portable_kernels.sh

[armwaheed]

executorch/backends/arm/scripts/build_portable_kernels.sh

Line 52 in f332196

echo "Error: Invalid toolchain selection, provided: ${tolchain}"

Please fix typo ${tolchain} and replace it with ${toolchain}

Thanks!

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[armwaheed]

Same issue in another file: #12859

[digantdesai]

FYI #12889