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/silvermont/string/sse4-memcmp-slm.S b/libc/arch-x86/silvermont/string/sse4-memcmp-slm.S
index e5028ff..f151168 100755
--- a/libc/arch-x86/silvermont/string/sse4-memcmp-slm.S
+++ b/libc/arch-x86/silvermont/string/sse4-memcmp-slm.S
@@ -76,7 +76,7 @@
 #endif
 
 #ifndef MEMCMP
-# define MEMCMP	memcmp
+# define MEMCMP	memcmp_sse4
 #endif
 
 #define CFI_PUSH(REG)	\