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/stdio/syscall.c

@@ -27,7 +27,7 @@
  * - `SYS_gettid`
  * - `SYS_getrandom`
  *
- * @return negative errno on error
+ * @return system call result, or -1 w/ errno
  */
 long syscall(long number, ...) {
   switch (number) {