Stack overflow checking with stack probes.

This adds a libcall name, a calling convention, and settings for
emitting stack probes, and implements them for x86 system_v ABIs.

lib/codegen/src/ir/mod.rs

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