kernel/log: Add log_str helper for custom log_* functions/overloads

When implementing custom log_... functions or custom overloads for the
core log functions like log_signal it is necessary to return `char *`
that are valid long enough.

The log_... functions implemented in log.cc use either `log_id_cache` or
`string_buf` which both are cleared on log_pop.

This commit adds a public `log_str` function which stores its argument
in the `log_id_cache` and returns the stored copy, such that custom
log functions outside of log.cc can also create strings that remain
valid until the next `log_pop`.

kernel/log.cc

@@ -662,6 +662,16 @@ const char *log_id(const RTLIL::IdString &str)
 	return p+1;
 }
 
+const char *log_str(const char *str)
+{
+	log_id_cache.push_back(strdup(str));
+	return log_id_cache.back();
+}
+
+const char *log_str(std::string const &str) {
+	return log_str(str.c_str());
+}
+
 void log_module(RTLIL::Module *module, std::string indent)
 {
 	std::stringstream buf;