Only rename mmap to mmap64 if we're L or newer.

Without this, setting `__USE_FILE_OFFSET64` and targeting pre-L made
mmap entirely unavailable.

Test: make checkbuild
Bug: android/ndk#332
Change-Id: I9f61c44f8d9ab5c7cae845c9f89a7d889c6df365

libc/include/sys/mman.h

@@ -49,7 +49,7 @@ __BEGIN_DECLS
 #define POSIX_MADV_WILLNEED   MADV_WILLNEED
 #define POSIX_MADV_DONTNEED   MADV_DONTNEED
 
-#if defined(__USE_FILE_OFFSET64)
+#if defined(__USE_FILE_OFFSET64) && __ANDROID_API__ >= 21
 void* mmap(void*, size_t, int, int, int, off_t) __RENAME(mmap64) __INTRODUCED_IN(21);
 #else
 void* mmap(void*, size_t, int, int, int, off_t);