[flang] add GETLOG runtime and extension implementation: get login username (#74628)

Get login username, ussage:
```
CHARACTER(32) :: login
CALL getlog(login)
WRITE(*,*) login
```
getlog is required for an exascale proxyapp.
https://proxyapps.exascaleproject.org/app/minismac2d/

https://github.com/Mantevo/miniSMAC/blob/f90446714226eeef650b78bce06ca4967792e74d/ref/smac2d.f#L615

https://github.com/Mantevo/miniSMAC/blob/f90446714226eeef650b78bce06ca4967792e74d/ref/smac2d.f#L1570



---------

Co-authored-by: Yi Wu <43659785+PAX-12-WU@users.noreply.github.com>
Co-authored-by: Yi Wu <yiwu02@wdev-yiwu02.arm.com>
Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>