[Flang] Make SLES 15 build tests (#87498)

SLES 15 comes with a GCC 7.5 as default, which does not support the
C++17 `<charconv>` header. This results in build errors when trying to
run `check-flang`.
This patch addresses that and uses the older `std::stol` for the string
-> number conversion to allow the SLES 15 buildbot
(https://lab.llvm.org/staging/#/builders/193) to turn green.