Emscripten: Xfail backtrace ui tests

It is possible to link libunwind and use the normal backtrace code, but it fails
to symbolize stack traces. I investigated and could get the list of instruction
pointers and symbol names, but I'm not sure how to use the dwarf info to map
from instruction pointer to source location. In any case, fixing this is
probably not a high priority.

See rust-lang#131738

tests/ui/backtrace/dylib-dep.rs

@@ -9,6 +9,7 @@
 //@ ignore-musl musl doesn't support dynamic libraries (at least when the original test was written).
 //@ needs-unwind
 //@ compile-flags: -g -Copt-level=0 -Cstrip=none -Cforce-frame-pointers=yes
+//@ ignore-emscripten Requires custom symbolization code
 //@ aux-crate: dylib_dep_helper=dylib-dep-helper.rs
 //@ aux-crate: auxiliary=dylib-dep-helper-aux.rs
 //@ run-pass

tests/ui/backtrace/line-tables-only.rs

@@ -10,6 +10,7 @@
 //@ compile-flags: -Cstrip=none -Cdebuginfo=line-tables-only
 //@ ignore-android FIXME #17520
 //@ ignore-fuchsia Backtraces not symbolized
+//@ ignore-emscripten Requires custom symbolization code
 //@ needs-unwind
 //@ aux-build: line-tables-only-helper.rs