Which is impossible with ISO C alone, so it will either stay Assembly, just written inline, or replaced by CPU compiler specific intrinsics, yet another GCC C feature.
It's true that some of it will remain assembly forever, but there are large chunks of existing assembly in the kernel that can be ported to regular C and where the current code paths are getting too long to be effectively maintainable in assembly - you can split out the parts that specifically touch magic registers, etc etc and keep those in asm while moving more complex logic into C.