RISC-V: Linux: Imply Zicntr from the IMA base behavior

As the author confirmed as in:
<https://lists.infradead.org/pipermail/linux-riscv/2025-May/070844.html>,
runtime detection of the Zicntr extension (as in the Linux kernel 6.15)
is currently (and technically) redundant on the current base IMA behavior
(although can be meaningful if new base behavior is added).

This commit implies the Zicntr extension from the base IMA behavior.

Author: a4lg

stdarch/crates/std_detect/src/detect/os/linux/riscv.rs

@@ -197,18 +197,16 @@ pub(crate) fn detect_features() -> cache::Initializer {
         // Query whether "I" base and extensions "M" and "A" (as in the ISA
         // manual version 2.2) are enabled.  "I" base at that time corresponds
         // to "I", "Zicsr", "Zicntr" and "Zifencei" (as in the ISA manual version
-        // 20240411) and we chose to imply "Zicsr" and "Zifencei" (not "Zicntr")
-        // because there will be a separate RISCV_HWPROBE_EXT_ZICNTR constant to
-        // determine existence of the "Zicntr" extension in Linux 6.15 (as of rc1).
-        // "fence.i" ("Zifencei") is conditionally valid on the Linux userland
-        // (when CMODX is enabled).
-        // This is a requirement of `RISCV_HWPROBE_KEY_IMA_EXT_0`-based tests.
+        // 20240411).
+        // This is a current requirement of
+        // `RISCV_HWPROBE_KEY_IMA_EXT_0`-based tests.
         let has_ima = (out[0].key != -1) && (out[0].value & RISCV_HWPROBE_BASE_BEHAVIOR_IMA != 0);
         if !has_ima {
             break 'hwprobe;
         }
         has_i |= has_ima;
         enable_feature(Feature::zicsr, has_ima);
+        enable_feature(Feature::zicntr, has_ima);
         enable_feature(Feature::zifencei, has_ima);
         enable_feature(Feature::m, has_ima);
         enable_feature(Feature::a, has_ima);