hexagon: Implement __clear_cache using inline assembly

Implement `__clear_cache` for Hexagon targets in Rust. Hexagon has
separate instruction and data caches, so this flushes dirty data cache
lines with `dccleaninva`, invalidates stale instruction cache lines
with `icinva`, then issues an `isync` barrier.

Based on the compiler-rt implementation from
llvm/llvm-project#188411.

library/compiler-builtins/compiler-builtins/src/hexagon.rs

@@ -1,5 +1,49 @@
 use core::arch::global_asm;
 
+// Hexagon L1 cache line size in bytes (Hexagon PRM sections 5.10.3-5.10.4).
+const CACHE_LINE_SIZE: usize = 32;
+
+intrinsics! {
+    pub unsafe extern "C" fn __clear_cache(start: *mut u8, end: *mut u8) {
+        // Hexagon has separate instruction and data caches.
+        let mask = !(CACHE_LINE_SIZE - 1);
+        let start_line = start.addr() & mask;
+        let end_addr = end.addr();
+
+        // Clean and invalidate data cache to push new code to memory and
+        // invalidate stale lines in the L2 cache.
+        let mut addr = start_line;
+        while addr < end_addr {
+            unsafe {
+                core::arch::asm!(
+                    "dccleaninva({addr})",
+                    addr = in(reg) addr,
+                    options(nostack, preserves_flags),
+                );
+            }
+            addr += CACHE_LINE_SIZE;
+        }
+
+        // Invalidate instruction cache so it re-fetches from memory.
+        addr = start_line;
+        while addr < end_addr {
+            unsafe {
+                core::arch::asm!(
+                    "icinva({addr})",
+                    addr = in(reg) addr,
+                    options(nostack, preserves_flags),
+                );
+            }
+            addr += CACHE_LINE_SIZE;
+        }
+
+        // Instruction sync barrier ensures subsequent fetches see the new code.
+        unsafe {
+            core::arch::asm!("isync", options(nostack, preserves_flags));
+        }
+    }
+}
+
 global_asm!(include_str!("hexagon/func_macro.s"), options(raw));
 
 global_asm!(