Fix handling of libs with a ro map and a rx map.

When the linker was changed so that it put a read-only map and
read-execute map, this code incorrectly computed the relative pc and
offset. Modify to look for the actual start map in this case.

Also, change the load_base name to load_bias.

Bug: 120613266

Test: Dumped logcat while dumping backtraces.

Change-Id: I6628694c5222ea34d63217af3d138707d4900004
diff --git a/libc/malloc_debug/backtrace.cpp b/libc/malloc_debug/backtrace.cpp
index cd8c334..0e3a53f 100644
--- a/libc/malloc_debug/backtrace.cpp
+++ b/libc/malloc_debug/backtrace.cpp
@@ -156,8 +156,8 @@
     }
 
     char offset_buf[128];
-    if (entry != nullptr && entry->offset != 0) {
-      snprintf(offset_buf, sizeof(offset_buf), " (offset 0x%" PRIxPTR ")", entry->offset);
+    if (entry != nullptr && entry->elf_start_offset != 0) {
+      snprintf(offset_buf, sizeof(offset_buf), " (offset 0x%" PRIxPTR ")", entry->elf_start_offset);
     } else {
       offset_buf[0] = '\0';
     }