Use the target-lexicon crate.

This switches from a custom list of architectures to use the
target-lexicon crate.

 - "set is_64bit=1; isa x86" is replaced with "target x86_64", and
   similar for other architectures, and the `is_64bit` flag is removed
   entirely.

 - The `is_compressed` flag is removed too; it's no longer being used to
   control REX prefixes on x86-64, ARM and Thumb are separate
   architectures in target-lexicon, and we can figure out how to
   select RISC-V compressed encodings when we're ready.

cranelift/filetests/isa/riscv/parse-encoding.cton

@@ -1,6 +1,6 @@
 ; Test the parser's support for encoding annotations.
 test legalizer
-isa riscv
+target riscv32
 
 function %parse_encoding(i32 [%x5]) -> i32 [%x10] {
     ; check: function %parse_encoding(i32 [%x5], i32 link [%x1]) -> i32 [%x10], i32 link [%x1] fast {