Implement POSIX lockf.

This has been requested a few times over the years. This is basically
a very late rebase of https://android-review.googlesource.com/45470
which was abandoned years ago. One addition is that this version has
_FILE_OFFSET_BITS=64 support.

POSIX puts this in <unistd.h>. glibc also has it in <fcntl.h>.

Bug: http://b/13077650
Change-Id: I5862b1dc326e326c01ad92438ecc1578d19ba739
diff --git a/libc/libc.mips64.map b/libc/libc.mips64.map
index e5933ae..fbeaa0d 100644
--- a/libc/libc.mips64.map
+++ b/libc/libc.mips64.map
@@ -1159,6 +1159,8 @@
     getifaddrs;
     if_freenameindex;
     if_nameindex;
+    lockf;
+    lockf64;
     preadv;
     preadv64;
     pthread_barrierattr_destroy;