[OE-core] [oe-core] why recompile gcc-cross/eglibc when update linux package?

[Yi Qingliang]

I update the kernel from 3.6.6 to 3.6.9, and recompile it: bitbake linux

but it will recompile eglibc/gcc-cross first, so long time:( why?
the eglibc will use kernel header? if so, understand.
but what about gcc-cross?

Yi Qingliang
Nanjing Jilong