scripts/sorttable: riscv: fix undeclared identifier 'EM_RISCV' error

Fix the following build failure reported in [1] by adding a conditional
definition of EM_RISCV in order to allow cross-compilation on machines
which do not have EM_RISCV definition in their host.

   scripts/sorttable.c:352:7: error: use of undeclared identifier 'EM_RISCV'

EM_RISCV was added to <elf.h> in glibc 2.24 so builds on systems with
glibc headers < 2.24 should show this error.

[mkubecek@suse.cz: changelog addition]
Link: https://lore.kernel.org/lkml/e8965b25-f15b-c7b4-748c-d207dda9c8e8@i2se.com/ [1]
Link: https://lkml.kernel.org/r/20210913030625.4525-1-miles.chen@mediatek.com


Fixes: 54fed35f ("riscv: Enable BUILDTIME_TABLE_SORT")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Cc: Michal Kubecek <mkubecek@suse.cz>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>