Rollup merge of #124741 - nebulark:patchable-function-entries-pr, r=estebank,workingjubilee

patchable-function-entry: Add unstable compiler flag and attribute

Tracking issue: #123115

Add the -Z patchable-function-entry compiler flag and the #[patchable_function_entry(prefix_nops = m, entry_nops = n)] attribute.
Rebased and adjusted the canditate implementation to match changes in the RFC.

compiler/rustc_span/src/symbol.rs

@@ -768,6 +768,7 @@ symbols! {
         enable,
         encode,
         end,
+        entry_nops,
         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_nops,
         preg,
         prelude,
         prelude_import,