Add backtrace_string and export to libmemunreachable

Add backtrace_string to convert a malloc_debug backtrace to a string.
Also move the backtrace functions to libc_malloc_debug_backtrace so that
libmemunreachable can reuse them.

Change-Id: I5ad67001c0b4d184903c762863a8588181d4873b
diff --git a/libc/private/libc_logging.h b/libc/private/libc_logging.h
index da2192b..e389565 100644
--- a/libc/private/libc_logging.h
+++ b/libc/private/libc_logging.h
@@ -101,6 +101,8 @@
 __LIBC_HIDDEN__ int __libc_format_log_va_list(int priority, const char* tag, const char* format,
                                               va_list ap);
 
+__LIBC_HIDDEN__ int __libc_write_log(int priority, const char* tag, const char* msg);
+
 //
 // Event logging.
 //