While in theory multiple unwinders could be compiled in, it does
not make sense in practise. Use a choice to make the unwinder
selection mutually exclusive and mandatory.

Already before this commit it has not been possible to deselect
FRAME_POINTER. Remove the obsolete comment.

Furthermore, to produce a meaningful backtrace with FRAME_POINTER
enabled the kernel needs a specific function prologue:
    mov    ip, sp
    stmfd    sp!, {fp, ip, lr, pc}
    sub    fp, ip, #4

To get to the required prologue gcc uses apcs and no-sched-prolog.
This compiler options are not available on clang, and clang is not
able to generate the required prologue. Make the FRAME_POINTER
config symbol depending on !clang.
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>