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.

lib/codegen/src/settings.rs

@@ -361,13 +361,11 @@ mod tests {
             "[shared]\n\
              opt_level = \"default\"\n\
              enable_verifier = true\n\
-             is_64bit = false\n\
              call_conv = \"fast\"\n\
              is_pic = false\n\
              colocated_libcalls = false\n\
              return_at_end = false\n\
              avoid_div_traps = false\n\
-             is_compressed = false\n\
              enable_float = true\n\
              enable_nan_canonicalization = false\n\
              enable_simd = true\n\