[flang] Handle missing LOGIN_NAME_MAX definition in runtime (#77775)

18af032c broke the Solaris build:
```
/vol/llvm/src/llvm-project/dist/flang/runtime/extensions.cpp:60:24: error: use of undeclared identifier 'LOGIN_NAME_MAX'
   60 |   const int nameMaxLen{LOGIN_NAME_MAX + 1};
      |                        ^
/vol/llvm/src/llvm-project/dist/flang/runtime/extensions.cpp:61:12: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
   61 |   char str[nameMaxLen];
      |            ^~~~~~~~~~
/vol/llvm/src/llvm-project/dist/flang/runtime/extensions.cpp:61:12: note: initializer of 'nameMaxLen' is unknown
/vol/llvm/src/llvm-project/dist/flang/runtime/extensions.cpp:60:13: note: declared here
   60 |   const int nameMaxLen{LOGIN_NAME_MAX + 1};
      |             ^
```
`flang/unittests/Runtime/CommandTest.cpp` has the same issue.

As documented in Solaris 11.4 `limits.h(3HEAD)`, `LOGIN_NAME_MAX` can be
undefined. To determine the value, `sysconf(3C)` needs to be used
instead.

Beside that portable method, Solaris also provides a non-standard
`LOGNAME_MAX` which could be used, but I've preferred the standard route
instead which would support other targets with the same issue.

Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.