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/pwrite.c

@@ -85,4 +85,4 @@ ssize_t pwrite(int fd, const void *buf, size_t size, int64_t offset) {
   return rc;
 }
 
-__strong_reference(pwrite, pwrite64);
+__weak_reference(pwrite, pwrite64);