Add dontthrow attribute to most libc functions

This will help C++ code that uses exceptions to be tinier. For example,
this change shaves away 1000 lines of assembly code from LLVM's libcxx,
which is 0.7% of all assembly instructions in the entire library.

libc/calls/statvfs.c

@@ -36,4 +36,4 @@ int statvfs(const char *path, struct statvfs *sv) {
   }
 }
 
-__strong_reference(statvfs, statvfs64);
+__weak_reference(statvfs, statvfs64);