Refactor calling convention settings. (#304)

Add a calling-convention setting to the `Flags` used as part of the
`TargetIsa`. This allows Cretonne code that generates calls to use the
correct convention, such as when emitting libcalls during legalization
or when the wasm frontend is decoding functions. This setting can be
overridden per-function.

This also adds "fast", "cold", and "fastcall" conventions, with "fast"
as the new default. Note that "fast" and "cold" are not intended to be
ABI-compatible across Cretonne versions.

This will also ensure Windows users will get an `unimplemented!` rather
than silent calling-convention mismatches, which reflects the fact that
Windows calling conventions are not yet implemented.

This also renames SpiderWASM, which isn't camel-case, to Baldrdash,
which is, and which is also a more relevant name.

cranelift/filetests/regalloc/unreachable_code.cton

@@ -6,7 +6,7 @@ isa x86 haswell
 
 ; This function contains unreachable blocks which trip up the register
 ; allocator if they don't get cleared out.
-function %unreachable_blocks(i64 vmctx) -> i32 spiderwasm {
+function %unreachable_blocks(i64 vmctx) -> i32 baldrdash {
 ebb0(v0: i64):
     v1 = iconst.i32 0
     v2 = iconst.i32 0