Add well-known names for runtime library functions.

Add a LibCall type which represents runtime library functions that many
be synthesized by Cretonne from pure instructions.

Add a LibCall variant to ExternalName to represent one of these runtime
functions.

lib/cretonne/src/ir/mod.rs

@@ -15,6 +15,7 @@ mod extfunc;
 mod extname;
 mod globalvar;
 mod heap;
+mod libcall;
 mod memflags;
 mod progpoint;
 mod sourceloc;
@@ -33,6 +34,7 @@ pub use ir::heap::{HeapData, HeapStyle, HeapBase};
 pub use ir::instructions::{Opcode, InstructionData, VariableArgs, ValueList, ValueListPool};
 pub use ir::jumptable::JumpTableData;
 pub use ir::layout::Layout;
+pub use ir::libcall::LibCall;
 pub use ir::memflags::MemFlags;
 pub use ir::progpoint::{ProgramPoint, ProgramOrder, ExpandedProgramPoint};
 pub use ir::sourceloc::SourceLoc;