#pragma STDC FENV_ROUND FE_TONEAREST gives misleading warning

The statement:
#pragma STDC FENV_ROUND FE_TONEAREST

Gets:
warning: expected identifier in '#pragma FENV_ROUND' - ignored [-Wignored-pragmas]
which is misleading.

1) The 'STDC' got dropped in the message.
2) FE_TONEAREST is a valid 'direction' as per the C standard.

If the pragma is not supported, the warning should say so.
If the pragma is supported, there should be no warning.

Start by searching the compiler diagnostics for “expected identifier in '#pragma FENV_ROUND'” and the handling of #pragma STDC FENV_ROUND. Check the relevant compiler tests, if found alongside that implementation. Done means the diagnostic preserves STDC and accurately reports unsupported pragmas, or the valid FE_TONEAREST form is accepted without a warning.

Written by the indexing model from the issue text.