Switch to upstream FreeBSD ldexp.c.

(Yes, this should be in libm, but it isn't. For once, this mistake predates
bionic!)

Change-Id: I1340407acbcdc0151dee969a38ba490bdd5e6767
diff --git a/libc/Android.mk b/libc/Android.mk
index 4d85f89..a41b1d4 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -58,7 +58,6 @@
     bionic/ioctl.c \
     bionic/isatty.c \
     bionic/issetugid.c \
-    bionic/ldexp.c \
     bionic/md5.c \
     bionic/memmem.c \
     bionic/pathconf.c \
@@ -230,6 +229,7 @@
     bionic/wchar.cpp \
 
 libc_upstream_freebsd_src_files := \
+    upstream-freebsd/lib/libc/gen/ldexp.c \
     upstream-freebsd/lib/libc/gen/sleep.c \
     upstream-freebsd/lib/libc/gen/usleep.c \
     upstream-freebsd/lib/libc/stdio/fclose.c \