Try to use __builtin_ffsl if ffsl is unavailable.

Some platforms (like those using Newlib) don't have ffs/ffsl.  This
commit adds a check to configure.ac for __builtin_ffsl if ffsl isn't
found.  __builtin_ffsl performs the same function as ffsl, and has the
added benefit of being available on any platform utilizing
Gcc-compatible compiler.

This change does not address the used of ffs in the MALLOCX_ARENA()
macro.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -152,6 +152,13 @@
 /* TLS is used to map arenas and magazine caches to threads. */
 #undef JEMALLOC_TLS
 
+/*
+ * ffs()/ffsl() functions to use for bitmapping.  Don't use these directly;
+ * instead, use jemalloc_ffs() or jemalloc_ffsl() from util.h.
+ */
+#undef JEMALLOC_INTERNAL_FFSL
+#undef JEMALLOC_INTERNAL_FFS
+
 /*
  * JEMALLOC_IVSALLOC enables ivsalloc(), which verifies that pointers reside
  * within jemalloc-owned chunks before dereferencing them.