[Bug libstdc++/125705] New: [libstdc++] `std::exit` does not call `terminate` when `atexit` handler exits via an exception

[hstong at ca dot ibm.com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125705

            Bug ID: 125705
           Summary: [libstdc++] `std::exit` does not call `terminate` when
                    `atexit` handler exits via an exception
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hstong at ca dot ibm.com
  Target Milestone: ---

Since at least C++98, `std::exit` has been specified such that `std::terminate`
is called when an `atexit`-registered function exits via an exception (C++98
subclause 18.3 [lib.support.start.term] paragraph 5,
https://wg21.link/support.start.term#9.1). libstdc++ does not meet this
requirement as it allows both the search for a handler and stack unwinding
itself to continue past the body of `std::exit`.

https://godbolt.org/z/18cPzKnTo

### Source (`<stdin>`)
```cpp
#include <stdlib.h>

void cleanup() { throw 42; }
void f [[gnu::weak]]() { exit(EXIT_FAILURE); }

volatile auto fn = cleanup;

int main(void) {
  atexit(fn);
  try {
    f();
  } catch (...) { return 0; }
}
```

### Compiler invocation
```
g++ -std=c++26 -xc++ - -o prog
```

### Program invocation
```
./prog
```

### Actual program output
(rc=0)

### Expected program output
```
terminate called after throwing an instance of 'int'
```

### Compiler version info (g++ -v)
```
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/17.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20260604 (experimental) (GCC)
```