fix Issue 22305 - ImportC: #pragma STDC FENV_ACCESS is not supported (#3212)

Author: WalterBright

spec/importc.dd

@@ -168,6 +168,22 @@ $(H2 $(LNAME2 implementation, Implementation))
     $(RATIONALE Implicit function declarations are very error-prone and cause hard
     to find bugs.)
 
+    $(H3 $(LNAME2 pragma-STDC-FENV_ACCESS, #pragma STDC FENV_ACCESS))
+
+    $(P This is described in C11 7.6.1)
+
+$(GRAMMAR
+#pragma STDC FENV_ACCESS on-off-switch
+
+on-off-switch:
+    ON
+    OFF
+    DEFAULT
+)
+
+    $(P It is completely ignored.)
+
+
 $(H2 $(LNAME2 limitations, Limitations))
 
     $(H3 $(LNAME2 exceptions, Exception Handling))