build: use weak-intrinsics compiler-builtins feature

this is needed to prevent conflicting with libgcc when using rust in the kernel. compiler-builtins itself is an unconditional dependency of core, but can be configured to emit weak symbols which should be dropped at link time when libgcc provides the same symbols

Bug: 304850822
Change-Id: Id405bdf3eb5a78450c6e4f94c5403bd1108b1922