Support DW_FORM_strx3

Already upstreaming this change at:
https://github.com/google/bloaty/pull/286, but it's faster
to land this here first.

Change-Id: I2033f5dc98543d80881570d812aef8967ec07dfc
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/bloaty/+/578983
Reviewed-by: Anthony Fandrianto <atyfto@google.com>
Commit-Queue: Yifei Teng <yifeit@google.com>
Fuchsia-Auto-Submit: Yifei Teng <yifeit@google.com>
diff --git a/src/dwarf/attr.cc b/src/dwarf/attr.cc
index 6cb359c..b794fd1 100644
--- a/src/dwarf/attr.cc
+++ b/src/dwarf/attr.cc
@@ -125,6 +125,8 @@
       return AttrValue::UnresolvedString(form, ReadFixed<uint8_t>(data));
     case DW_FORM_strx2:
       return AttrValue::UnresolvedString(form, ReadFixed<uint16_t>(data));
+    case DW_FORM_strx3:
+      return AttrValue::UnresolvedString(form, ReadFixed<uint32_t, 3>(data));
     case DW_FORM_strx4:
       return AttrValue::UnresolvedString(form, ReadFixed<uint32_t>(data));
     case DW_FORM_strx: