arch: implement brute force find_lsb_set()

On RISC-V 64-bit, GCC complains about undefined reference
to 'ffs' via __builtin_ffs(). So implement a brute force
way to do it. Once the toolchain has __builtin_ffs(),
this can be reverted.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/Kconfig

@@ -955,3 +955,9 @@ config BOARD
 	  The Board is the first location where we search for a linker.ld file,
 	  if not found we look for the linker file in
 	  soc/<arch>/<family>/<series>
+
+config TOOLCHAIN_HAS_BUILTIN_FFS
+	bool
+	default y if !(64BIT && RISCV)
+	help
+	  Hidden option to signal that toolchain has __builtin_ffs*().

include/arch/common/ffs.h

@@ -52,7 +52,32 @@ static ALWAYS_INLINE unsigned int find_msb_set(uint32_t op)
 
 static ALWAYS_INLINE unsigned int find_lsb_set(uint32_t op)
 {
+#ifdef CONFIG_TOOLCHAIN_HAS_BUILTIN_FFS
 	return __builtin_ffs(op);
+
+#else
+	/*
+	 * Toolchain does not have __builtin_ffs().
+	 * Need to do this manually.
+	 */
+	int bit;
+
+	if (op == 0) {
+		return 0;
+	}
+
+	for (bit = 0; bit < 32; bit++) {
+		if ((op & (1 << bit)) != 0) {
+			return (bit + 1);
+		}
+	}
+
+	/*
+	 * This should never happen but we need to keep
+	 * compiler happy.
+	 */
+	return 0;
+#endif /* CONFIG_TOOLCHAIN_HAS_BUILTIN_FFS */
 }
 
 #ifdef __cplusplus