riscv: Gate FUNCTION_ALIGNMENT_4B on DYNAMIC_FTRACE

The FUNCTION_ALIGNMENT_4B select forces the whole kernel to be built
with -fmin-function-alignment=4. This alignment is only needed so the
patchable-function-entry NOPs, which arch/riscv/Makefile emits under
CONFIG_DYNAMIC_FTRACE, can be patched reliably on RISCV_ISA_C=y builds
where compressed instructions otherwise allow 2-byte function
alignment.

The select is currently gated on HAVE_DYNAMIC_FTRACE, a capability bit
that is selected whenever the toolchain supports dynamic ftrace, rather
than on whether tracing is actually enabled. As a result every
RISCV_ISA_C=y build gets 4-byte function alignment across the entire
kernel even when function tracing is disabled, needlessly growing the
kernel image and wasting instruction cache for a feature that is not
in use.

Gate the select on DYNAMIC_FTRACE instead, matching the condition under
which arch/riscv/Makefile emits -fpatchable-function-entry, so the
alignment is only applied when it is actually needed.

Fixes: c41bf43 ("riscv: ftrace: align patchable functions to 4 Byte boundary")
Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Link: https://patch.msgid.link/20260706130415.463682-1-qirui.001@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: qiruibd

arch/riscv/Kconfig

@@ -157,7 +157,7 @@ config RISCV
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
-	select FUNCTION_ALIGNMENT_4B if HAVE_DYNAMIC_FTRACE && RISCV_ISA_C
+	select FUNCTION_ALIGNMENT_4B if DYNAMIC_FTRACE && RISCV_ISA_C
 	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS if HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
 	select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if (DYNAMIC_FTRACE_WITH_ARGS && !CFI)
 	select HAVE_DYNAMIC_FTRACE_WITH_ARGS if HAVE_DYNAMIC_FTRACE