Use ifunc to dynamically dispatch libc routines for x86

Test: run bionic unit test in aosp_cf_x86_phone emulator
Change-Id: Ib0c0de37cd38d24bfce2dfbe35b8fd8edff004af
diff --git a/libc/arch-x86/atom/string/sse2-strlen-atom.S b/libc/arch-x86/atom/string/sse2-strlen-atom.S
index 81768fb..6a1acfb 100644
--- a/libc/arch-x86/atom/string/sse2-strlen-atom.S
+++ b/libc/arch-x86/atom/string/sse2-strlen-atom.S
@@ -31,7 +31,7 @@
 #ifndef USE_AS_STRCAT
 
 # ifndef STRLEN
-#  define STRLEN strlen
+#  define STRLEN strlen_atom
 # endif
 
 # ifndef L