Remove bb_strlen() in favor of -fno-builtin-strlen.  Saves as many bytes
as the old optimization did (actually does slightly better under gcc 4.0), and
simplifies the code.
diff --git a/Rules.mak b/Rules.mak
index 95a6714..c3ae258 100644
--- a/Rules.mak
+++ b/Rules.mak
@@ -125,6 +125,7 @@
 # Pin CHECKED_CFLAGS with := so it's only evaluated once.
 CHECKED_CFLAGS:=$(call check_gcc,-funsigned-char,)
 CHECKED_CFLAGS+=$(call check_gcc,-mmax-stack-frame=256,)
+CHECKED_CFLAGS+=$(call check_gcc,-fno-builtin-strlen)
 
 # Preemptively pin this too.
 PROG_CFLAGS:=