refactor(esp): split _exit into separate file for QEMU support

Move _exit implementation from syscalls.c to syscalls_exit.c to allow
easy replacement with semihosting version for QEMU emulator support.

This separation enables llgo to compile the semihosting _exit as a
separate object file that can override the default implementation
when building for QEMU targets.

Related to: goplus/llgo#1539

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: luoliwoshang

libgloss/riscv/esp/syscalls.c

@@ -74,18 +74,6 @@ _getpid (void)
     return -1;
 }
 
-void
-__attribute__ ((noreturn))
-_exit (int status)
-{
-    // refer to esp-idf, use an invalid instruction to make it panic 
-    asm("unimp");
-
-    for (;;) {
-        ;
-    }
-}
-
 int
 _open (const char *file, int flags, int mode)
 {

libgloss/riscv/esp/syscalls_exit.c

@@ -0,0 +1,16 @@
+/* Default _exit implementation for ESP32 RISC-V chips.
+ * This is separated from syscalls.c to allow easy replacement
+ * with semihosting version for QEMU emulator support.
+ */
+
+void
+__attribute__ ((noreturn))
+_exit (int status)
+{
+    // refer to esp-idf, use an invalid instruction to make it panic
+    asm("unimp");
+
+    for (;;) {
+        ;
+    }
+}