libia2: Replace libc backtrace with libunwind

backtrace(3) doesn't support unwinding through PLT stubs so this commit
replaces it with libunwind. Since we want backtraces on by default I'm not sure
if this is exactly what we want but it's good enough for now.

runtime/libia2/CMakeLists.txt

@@ -19,6 +19,7 @@ endif()
 
 target_link_options(libia2
     INTERFACE
+        "-lunwind"
         "-pthread"
         "-Wl,--wrap=pthread_create"
         "-Wl,--wrap=main"
@@ -29,6 +30,10 @@ target_link_options(libia2
         "-Wl,-z,relro"
 )
 
+if (LIBIA2_X86_64)
+    target_link_options(libia2 INTERFACE "-lunwind-x86_64")
+endif()
+
 target_link_libraries(libia2 PRIVATE dl)
 
 target_include_directories(libia2 PUBLIC include)

runtime/libia2/ia2.c

@@ -1,7 +1,7 @@
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
-#include <execinfo.h>
+#include <libunwind.h>
 #include <inttypes.h>
 #include <stdio.h>
 #include <string.h>
@@ -511,15 +511,21 @@ int protect_pages(struct dl_phdr_info *info, size_t size, void *data) {
 
 #if IA2_DEBUG
 void ia2_print_backtrace(void) {
-#define IA2_BT_SZ 10
-    void *ra_buf[IA2_BT_SZ];
-    int num_ra = backtrace(ra_buf, IA2_BT_SZ);
-    char **fn_names = backtrace_symbols(ra_buf, num_ra);
-    for (size_t i = 0; i < num_ra; i++) {
-        if (fn_names) {
-            fprintf(stderr, "#%d %p in %s ()\n", i, ra_buf[i], fn_names[i]);
+    unw_cursor_t cursor;
+    unw_context_t uc;
+    unw_word_t pc;
+
+    unw_getcontext(&uc);
+    unw_init_local(&cursor, &uc);
+    int i = 0;
+    while (unw_step(&cursor) > 0) {
+        unw_get_reg(&cursor, UNW_REG_IP, &pc);
+        Dl_info dlinf = {0};
+        bool found = dladdr((void *)pc, &dlinf) != 0;
+        if (found) {
+            fprintf(stderr, "#%d %p in %s ()\n", i++, (void *)pc, dlinf.dli_sname);
         } else {
-            fprintf(stderr, "#%d %p ()\n", i, ra_buf[i]);
+            fprintf(stderr, "#%d %p ()\n", i++, (void *)pc);
         }
     }
     fflush(stderr);