Make dlerror(3) thread-safe.

I gave up trying to use the usual thread-local buffer idiom; calls to
calloc(3) and free(3) from any of the "dl" functions -- which live in
the dynamic linker -- end up resolving to the dynamic linker's stubs.
I tried to work around that, but was just making things more complicated.
This alternative costs us a well-known TLS slot (instead of the
dynamically-allocated TLS slot we'd have used otherwise, so no difference
there), plus an extra buffer inside every pthread_internal_t.

Bug: 5404023
Change-Id: Ie9614edd05b6d1eeaf7bf9172792d616c6361767

libc/bionic/strsignal.cpp

@@ -42,5 +42,5 @@ char* strsignal(int signal_number) {
   }
 
   LOCAL_INIT_THREAD_LOCAL_BUFFER(char*, strsignal, NL_TEXTMAX);
-  return const_cast<char*>(__strsignal(signal_number, strsignal_buffer, strsignal_buffer_size));
+  return const_cast<char*>(__strsignal(signal_number, strsignal_tls_buffer, strsignal_tls_buffer_size));
 }