Support for -Z patchable-function-entry

`-Z patchable-function-entry` works like `-fpatchable-function-entry`
on clang/gcc. The arguments are total nop count and function offset.

See MCP rust-lang/compiler-team#704

compiler/rustc_session/src/config.rs

@@ -2963,7 +2963,7 @@ pub(crate) mod dep_tracking {
         CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FunctionReturn,
         InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
         LtoCli, NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes,
-        Polonius, RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm,
+        PatchableFunctionEntry, Polonius, RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm,
         SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
     };
     use crate::lint;
@@ -3071,6 +3071,7 @@ pub(crate) mod dep_tracking {
         OomStrategy,
         LanguageIdentifier,
         NextSolverConfig,
+        PatchableFunctionEntry,
         Polonius,
         InliningThreshold,
         FunctionReturn,
@@ -3248,6 +3249,32 @@ impl DumpMonoStatsFormat {
     }
 }
 
+/// `-Z patchable-function-entry` representation - how many nops to put before and after function
+/// entry.
+#[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]
+pub struct PatchableFunctionEntry {
+    /// Nops before the entry
+    prefix: u8,
+    /// Nops after the entry
+    entry: u8,
+}
+
+impl PatchableFunctionEntry {
+    pub fn from_nop_count_and_offset(nop_count: u8, offset: u8) -> Option<PatchableFunctionEntry> {
+        if nop_count < offset {
+            None
+        } else {
+            Some(Self { prefix: offset, entry: nop_count - offset })
+        }
+    }
+    pub fn prefix(&self) -> u8 {
+        self.prefix
+    }
+    pub fn entry(&self) -> u8 {
+        self.entry
+    }
+}
+
 /// `-Zpolonius` values, enabling the borrow checker polonius analysis, and which version: legacy,
 /// or future prototype.
 #[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]