xtensa: add kernel ABI selection to Kconfig

Add choice to use default or call0 ABI for the kernel code. If call0 ABI
is chosen add '-mabi=call0' to the flags. The toolchain support for this
option is rather new so only enable it when the compiler supports it.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
2 files changed