linux(4): Fix gcc buld.

gcc failed as it didn't inlined the builtins and generates calls to
the libgcc, ld can't find libgcc as cross-toolchain libgcc is not installed.
To avoid this add internal vDSO ffs functions without optimized builtins.

Reported by:		jhb
MFC after:		2 weeks

sys/amd64/linux/linux_vdso_gtod.c

@@ -36,7 +36,6 @@ __FBSDID("$FreeBSD$");
 #include <sys/vdso.h>
 #undef	_KERNEL
 #include <stdbool.h>
-#include <strings.h>
 
 #include <machine/atomic.h>
 #include <machine/stdarg.h>

sys/amd64/linux32/linux32_vdso_gtod.c

@@ -37,7 +37,6 @@ __FBSDID("$FreeBSD$");
 #include <sys/vdso.h>
 #undef	_KERNEL
 #include <stdbool.h>
-#include <strings.h>
 
 #include <machine/atomic.h>
 #include <machine/stdarg.h>

sys/arm64/linux/linux_vdso_gtod.c

@@ -38,7 +38,6 @@ __FBSDID("$FreeBSD$");
 #include <sys/vdso.h>
 #undef	_KERNEL
 #include <stdbool.h>
-#include <strings.h>
 
 #include <machine/atomic.h>
 #include <machine/stdarg.h>

sys/compat/linux/linux_vdso_gtod.inc

@@ -26,6 +26,40 @@
  * SUCH DAMAGE.
  */
 
+static int
+fls(int mask)
+{
+
+	if (mask == 0)
+		return (0);
+	return ((__builtin_clz(mask) ^ 0x1f) + 1);
+}
+
+#ifdef _LP64
+static int
+ffsl(long mask)
+{
+	int bit;
+
+	if (mask == 0)
+		return (0);
+	for (bit = 1; !(mask & 1); bit++)
+		mask = (unsigned long)mask >> 1;
+	return (bit);
+}
+#else
+static int
+ffsll(long long mask)
+{
+	int bit;
+
+	if (mask == 0)
+		return (0);
+	for (bit = 1; !(mask & 1); bit++)
+		mask = (unsigned long long)mask >> 1;
+	return (bit);
+}
+#endif
 
 static int
 __vdso_native_to_linux_timespec(struct l_timespec *lts,

sys/i386/linux/linux_vdso_gtod.c

@@ -36,7 +36,6 @@ __FBSDID("$FreeBSD$");
 #include <sys/vdso.h>
 #undef	_KERNEL
 #include <stdbool.h>
-#include <strings.h>
 
 #include <machine/atomic.h>
 #include <machine/stdarg.h>