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Replace HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR by a config option
named CONFIG_HAVE_FUNCTION_DESCRIPTORS and use it instead of
'dereference_function_descriptor' macro to know whether an
arch has function descriptors.

To limit churn in one of the following patches, use
an #ifdef/#else construct with empty first part
instead of an #ifndef in asm-generic/sections.h

On powerpc, make sure the config option matches the ABI used
by the compiler with a BUILD_BUG_ON() and add missing _CALL_ELF=2
when calling 'sparse' so that sparse sees the same piece of
code as GCC.

And include a helper to check whether an arch has function
descriptors or not : have_function_descriptors()
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4a0f11fb0ea74a3197bc44dd7ba25e53a24fd03d.1644928018.git.christophe.leroy@csgroup.eu