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/x86/binary32-float.cton

@@ -1,6 +1,6 @@
 ; Binary emission of 32-bit floating point code.
 test binemit
-isa x86 haswell
+target i686 haswell
 
 ; The binary encodings can be verified with the command:
 ;