Support #[patchable_function_entries]

See [RFC](https://github.com/maurer/rust-rfcs/blob/patchable-function-entry/text/0000-patchable-function-entry.md) (yet to be numbered)

TODO before submission:
* Needs an RFC
* Improve error reporting for malformed attributes

compiler/rustc_span/src/symbol.rs

@@ -768,6 +768,7 @@ symbols! {
         enable,
         encode,
         end,
+        entry,
         enumerate_method,
         env,
         env_CFG_RELEASE: env!("CFG_RELEASE"),
@@ -1383,6 +1384,7 @@ symbols! {
         passes,
         pat,
         pat_param,
+        patchable_function_entry,
         path,
         pattern_complexity,
         pattern_parentheses,
@@ -1421,6 +1423,7 @@ symbols! {
         prefetch_read_instruction,
         prefetch_write_data,
         prefetch_write_instruction,
+        prefix,
         preg,
         prelude,
         prelude_import,