libc: cleanup strchr

Move strchr to a .cpp file, and change to bionic directory.

Change-Id: I64ade7df326c0a9a714aca4caf5647b6833b1c97
diff --git a/libc/Android.mk b/libc/Android.mk
index eaeb6fc..21cd33e 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -66,7 +66,6 @@
 	string/index.c \
 	string/strcasecmp.c \
 	string/strcat.c \
-	string/strchr.c \
 	string/strcspn.c \
 	string/strdup.c \
 	string/strlcat.c \
@@ -238,6 +237,7 @@
     bionic/signalfd.cpp \
     bionic/sigwait.cpp \
     bionic/__strcat_chk.cpp \
+    bionic/strchr.cpp \
     bionic/__strcpy_chk.cpp \
     bionic/strerror.cpp \
     bionic/strerror_r.cpp \