-latomics vs riscv64

[Mathieu Malaterre <malat@debian.org>]

[cc me please]

Dear riscv64 porters,

I am trying to get JPEG-XL to compile on riscv64. After reading of:

* https://github.com/riscv-collab/riscv-gcc/issues/12

I am under the impression that the black magic to get -latomic passed
on the linker line is handled somewhere in gcc spec file.

This also seems to be the case, per my reading of the log file at:

* https://buildd.debian.org/status/fetch.php?pkg=jpeg-xl&arch=riscv64&ver=0.7.0%7Egit20220120.0647da4%2Bds-3&stamp=1643404075&raw=0

The following c++11 code (1):

```
#include <atomic>
#include <cstdint>
std::atomic<uint64_t> num (0);
int main() {
  uint64_t i = num.load(std::memory_order_relaxed);
  return 0;
}
```

seems to compile and link nicely without the need for an explicit
`-latomic`. However somewhere later during the compilation, I receive
an error that:

/usr/bin/ld: libjxl_dec.so.0.7.0: undefined reference to `__atomic_fetch_and_1'
/usr/bin/ld: libjxl_dec.so.0.7.0: undefined reference to `__atomic_fetch_or_1'

So now it seems that I need an explicit `-latomic`

Could someone please share his knowledge on -latomic on riscv64 Debian
arch ? Please note that the above c++11 code rightfully fails to
compile on armel/mipsel and powerpc which allow the buildsystem to
detect the need for an explicit `-latomic` being added by cmake.

Thanks,

(1) https://salsa.debian.org/debian-phototools-team/libjxl/-/blob/master/debian/patches/atomics.patch