builtin: improve musl/Alpine support (define weak backtrace/backtrace_symbols/backtrace_symbols_fd symbols) (#14250)

vlib/builtin/builtin_backtraces_d_musl.c.v

@@ -0,0 +1,20 @@
+module builtin
+
+// These are just dummy implementations to appease linking on musl/alpine
+
+[export: 'backtrace_symbols']
+[weak]
+fn vbacktrace_symbols(const_buffer &voidptr, size int) &&char {
+	return 0
+}
+
+[export: 'backtrace']
+[weak]
+fn vbacktrace(buffer &voidptr, size int) int {
+	return 0
+}
+
+[export: 'backtrace_symbols_fd']
+[weak]
+fn vbacktrace_symbols_fd(const_buffer &voidptr, size int, fd int) {
+}