configure/riscv: detect fast CLZ

RISC-V defines the CLZ instruction as part of the ratified Zbb subset
of the (not yet ratified) bit mapulation extension (B). We can detect
it from the __riscv_zbb predefined constant. At least GCC 12 already
supports this correctly.

Note that the macro will be non-zero if supported, zero if enabled
in the compiler flags (e.g. -march=rv64gzbb) but not known to the
compiler, and undefined otherwise.

configure

@@ -5334,6 +5334,12 @@ elif enabled ppc; then
         ;;
     esac
 
+elif enabled riscv; then
+
+    if test_cpp_condition stddef.h "__riscv_zbb"; then
+        enable fast_clz
+    fi
+
 elif enabled sparc; then
 
     case $cpu in