libroot: Replace custom ffs implementation with musl's.

It already has a per-arch implementation or a fallback in
musl's own arch_atomic.h, which we already imported so we
might as well leverage it.

src/system/libroot/posix/musl/misc/Jamfile

@@ -2,6 +2,7 @@ SubDir HAIKU_TOP src system libroot posix musl misc ;
 
 SubDirSysHdrs [ FDirName $(SUBDIR) .. include ] ;
 UseHeaders [ FDirName $(SUBDIR) .. internal ] ;
+UseHeaders [ FDirName $(SUBDIR) .. arch $(TARGET_KERNEL_ARCH_DIR) ] ;
 
 local architectureObject ;
 for architectureObject in [ MultiArchSubDirSetup ] {
@@ -10,6 +11,7 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 
 		MergeObject <$(architecture)>posix_musl_misc.o :
 			a64l.c
+			ffs.c
 			;
 	}
 }

src/system/libroot/posix/musl/misc/ffs.c

@@ -0,0 +1,6 @@
+#include "atomic.h"
+
+int ffs(int i)
+{
+	return i ? a_ctz_l(i)+1 : 0;
+}

src/system/libroot/posix/string/Jamfile

@@ -20,7 +20,6 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			bcmp.c
 			bcopy.c
 			bzero.c
-			ffs.cpp
 			memccpy.c
 			memchr.c
 			memcmp.c

src/system/libroot/posix/string/ffs.cpp

@@ -1,24 +0,0 @@
-/*
- * Copyright 2020, Adrien Destugues <[email protected]>.
- * Distributed under the terms of the MIT License.
- */
-
-
-// find first (least significant) set bit
-extern "C" int
-ffs(int value)
-{
-#ifdef __riscv
-	// TODO: As of this writing, gcc 8.x seems
-	// to have an issue with infinite recursion.
-	// Re-examine in future GCC updates
-	int bit;
-	if (value == 0)
-		return 0;
-	for (bit = 1; !(value & 1); bit++)
-		value >>= 1;
-	return bit;
-#else
-	return __builtin_ffs(value);
-#endif
-}