riscv: ftrace: select HAVE_BUILDTIME_MCOUNT_SORT

riscv: ftrace: select HAVE_BUILDTIME_MCOUNT_SORT

RISC-V already satisfies all prerequisites for build-time mcount sorting:
the sorttable host tool handles EM_RISCV in its machine-type dispatch, and
the __mcount_loc section entries are stored as direct virtual addresses in
the final vmlinux binary, so no relocation processing is required during
the sort step.

Select HAVE_BUILDTIME_MCOUNT_SORT so that BUILDTIME_MCOUNT_SORT is
automatically enabled when DYNAMIC_FTRACE is configured.  This allows
sorttable to sort the __mcount_loc section at link time, making the
run-time ftrace initialisation path skip the software sort and reducing
kernel startup overhead.

Verified with CONFIG_FTRACE_SORT_STARTUP_TEST=y, which confirms that
the section produced by the build is already in ascending order:

  [    0.000000] ftrace section at ffffffff81015a60 sorted properly

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Tested-by: Guo Ren <guoren@kernel.org>
Link: https://patch.msgid.link/20260409114736.907-1-cp0613@linux.alibaba.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>