Fix syscall(2) returning -errno instead of using POSIX errno scheme (#830)

As of now, the syscall function is implemented as alike to how the
linux kernel sycall ABI works, returning -errno upon errors without
setting the value of errno. However, this does not conform to the
expectations of most software, which expect it to return -1 and set
errno, which is how it works on other libcs, which document it
accordingly:

> The return value is defined by the system call being invoked.  In
> general, a 0 return value indicates success.  A -1 return value
> indicates an error, and an error number is stored in errno.
- Linux man-pages, syscall(2)

> The return value is the return value from the system call, unless
> the system call failed.  In that case, ‘syscall’ returns ‘-1’ and
> sets ‘errno’ to an error code that the system call returned.
- glibc manual, (libc)System Calls

> When the C-bit is set, syscall returns -1 and sets the external
> variable errno (see intro(2)).
- 4BSD manual, syscall(2)

> A -1 return value indicates an error, and an error code is stored in
> errno.
- 4.4BSD, FreeBSD, OpenBSD and NetBSD manuals (same quote is found in
  all of them), syscall(2)

This patch corrects the syscall function to work in the same way as in
other libcs.

libc/stdio/syscall.c

@@ -32,7 +32,8 @@
 long syscall(long number, ...) {
   switch (number) {
     default:
-      return -ENOSYS;
+      errno = ENOSYS;
+      return -1;
     case SYS_gettid:
       return gettid();
     case SYS_getrandom: {
@@ -43,7 +44,7 @@ long syscall(long number, ...) {
       unsigned flags = va_arg(va, unsigned);
       va_end(va);
       ssize_t rc = getrandom(buf, buflen, flags);
-      return rc == -1 ? -errno : rc;
+      return rc;
     }
   }
 }