kernel: Add kernel specific OBJDUMP

kernel: Add kernel specific OBJDUMP

This helps in switching toolchains cleanly for kernel build
between gcc and clang

Currently, some kernels allow building with clang but not all
the distro might use clang as default system compiler but kernel
may demand gcc which is provided via KERNEL_* variables, however
kernel does use OBJCOPY at places during build and it maybe set
to use llvm objcopy when using clang. That should be a deliberate
setting when clang is used for kernel as well, otherwise it should
use binutils provided objcopy

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>