kbuild: make it possible for the linker to discard local symbols from vmlinux

Make it possible for the linker to discard local symbols from vmlinux as
they cause vmlinux to balloon when CONFIG_KALLSYMS=y and they cause
dump_stack() and get_wchan() to produce useless information under some
circumstances.

With this we add a config option (CONFIG_STRIP_ASM_SYMS) that will cause
the build to supply -X to the linker to tell it to strip temporary local
symbols.

This doesn't seem to cause gdb any problems.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>