Add clobber-only register classes for asm!

These are needed to properly express a function call ABI using a clobber
list, even though we don't support passing actual values into/out of
these registers.

compiler/rustc_span/src/symbol.rs

@@ -752,6 +752,7 @@ symbols! {
         minnumf64,
         mips_target_feature,
         misc,
+        mmx_reg,
         modifiers,
         module,
         module_path,
@@ -894,6 +895,7 @@ symbols! {
         prefetch_read_instruction,
         prefetch_write_data,
         prefetch_write_instruction,
+        preg,
         prelude,
         prelude_import,
         preserves_flags,
@@ -1333,6 +1335,7 @@ symbols! {
         wrapping_mul,
         wrapping_sub,
         write_bytes,
+        x87_reg,
         xmm_reg,
         ymm_reg,
         zmm_reg,