19341: Picolibc updates r=benpicco a=keith-packard

### Contribution description

Minor updates to picolibc support for newer versions of picolibc, including 1.8.


### Testing procedure

Here's the current build result for a board I happen to have on my bench right now:

```
$ make -C examples/blinky BOARD=nucleo-f103rb FEATURES_REQUIRED=picolibc
...
/home/keithp/src/RIOT/sys/picolibc_syscalls_default/syscalls.c:319:1: error: unknown type name '_READ_WRITE_RETURN_TYPE'
  319 | _READ_WRITE_RETURN_TYPE read(int fd, void *dest, size_t count)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/home/keithp/src/RIOT/sys/picolibc_syscalls_default/syscalls.c:351:1: error: unknown type name '_READ_WRITE_RETURN_TYPE'
  351 | _READ_WRITE_RETURN_TYPE write(int fd, const void *src, size_t count)
      | ^~~~~~~~~~~~~~~~~~~~~~~
...
```

### Description of changes

 1. Fix the read/write return values (Picolibc 1.8 uses POSIX standard types now)
 2. Add new symbols to linker scripts (Picolibc needs help dealing with TLS alignment)
 3. Insert carriage return when sending newline to console.


19344: test/periph_rtc: reset struct tm time between tests r=benpicco a=kfessel

### Contribution description

while reviewing #19340 i found test/periph_rtc to be insufficient to prove rtc_set_time is working. this changes that and avoids accidental reuse of struct tm time and ms  values by resetting time and ms;

### Testing procedure

run the test

### Issues/PRs references

#19340

Co-authored-by: Keith Packard <keithp@keithp.com>
Co-authored-by: Karl Fessel <karl.fessel@ovgu.de>