riscv/ftrace: Export _mcount when DYNAMIC_FTRACE isn't set

The EXPORT_SYMBOL(_mcount) for RISC-V ended up inside a
CONFIG_DYNAMIC_FTRACE ifdef.  If you enable modules without enabling
CONFIG_DYNAMIC_FTRACE then you'll get a build error without this patch
because the modules won't be able to find _mcount.

The new behavior is to export _mcount whenever CONFIG_FUNCTION_TRACER is
defined.  This matches what every other architecture is doing.

Signed-off-by: Alan Kao <alankao@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Cc: Zong Li <zong@andestech.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>