Extract kernel compile from buildAotSnapshot (#17062)

Moves the kernel compile step to the beginning of the AOT build in a
separate method. This is pre-factoring for iOS universal builds where
the kernel build happens once, but we then snapshot twice: once for
armv7 and once for arm64.

This also writes dependencies to build/kernel_compile.d rather than
build/aot/snapshot.d, since that is immediately overwritten by
gen_snapshot.