riscv: Define PLATFORM__SET_BIT for generic_set_bit()

riscv bitops.h provides a __set_bit() but does not define PLATFORM__SET_BIT
as a result generic_set_bit() is used instead of the architecturally
provided __set_bit().

This patch defines PLATFORM__SET_BIT which means that __set_bit() in x86
bitops.h will be called whenever generic_set_bit() is called - as opposed
to the default cross-platform generic_set_bit().

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <green.hu@gmail.com>