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

tests/string_test.cpp

@@ -29,7 +29,7 @@ TEST(string, strerror) {
   ASSERT_STREQ("Unknown error 1234", strerror(1234));
 }
 
-void* ConcurrentStrErrorFn(void* arg) {
+static void* ConcurrentStrErrorFn(void* arg) {
   bool equal = (strcmp("Unknown error 2002", strerror(2002)) == 0);
   return reinterpret_cast<void*>(equal);
 }
@@ -88,7 +88,7 @@ TEST(string, strsignal) {
   ASSERT_STREQ("Unknown signal 1234", strsignal(1234)); // Too large.
 }
 
-void* ConcurrentStrSignalFn(void* arg) {
+static void* ConcurrentStrSignalFn(void* arg) {
   bool equal = (strcmp("Unknown signal 2002", strsignal(2002)) == 0);
   return reinterpret_cast<void*>(equal);
 }