I thought I guessed a fix in #130836, but I was wrong.
We actually had the same code in
`flang/cmake/modules/FlangCommon.cmake`.
The check does not pass in flang-rt bootstrap build, because
`-nostdinc++` is added for all `runtimes` checks.
I decided to make the check with the C header, though, I am still
unsure whether it is reliable with a clang that has not been
installed (it is taken from the build structure during flang-rt
configure step).
I verified that this PR enables REAL(16) math entries on aarch64.
c542991703