Try to improve performance by using a read/write buffer instead of I/O.

The calls to fwrite/fread can be very expensive. GCC avoids this by using a
buffer to read and write from the file, thus limiting the number of fwrite/fread
calls.

<rdar://problem/13466086>


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@181924 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/SDKs/linux/usr/include/stdio.h b/SDKs/linux/usr/include/stdio.h
index 7c258d2..0ca10fa 100644
--- a/SDKs/linux/usr/include/stdio.h
+++ b/SDKs/linux/usr/include/stdio.h
@@ -38,5 +38,6 @@
 extern size_t fread(void * restrict, size_t, size_t, FILE * restrict);
 extern long ftell(FILE *);
 extern int fseek(FILE *, long, int);
+extern void setbuf(FILE * restrict, char * restrict);
 
 #endif /* __STDIO_H__ */