fix: handle situations where linked binary does not link against a libc flavor

We have seen in the Dash0 operator that there are situations in which a
dynamically linked binary does not actually link *also* to a libc flavor
(in our case it was a Go binaru with cgo disable, with the flag
`-buildmode=pie` set at build time.

THis causes the linker to fail because the weak symbol __environ required
by the injector did not exist.

This commit provides a fallback weak __environ symbol, which will be
overwritten by the strong __environ symbol of any libc flavor.

Author: mmanciop

autoinstrumentation/injector/src/injector.c

@@ -12,6 +12,10 @@
   "-javaagent:"                                                          \
   "/otel-auto-instrumentation-injector/instrumentation/jvm/javaagent.jar"
 
+// Provide a fallback weak symbol, in case the linked binary
+// does not actually link against a libc flavor.
+__attribute__((weak)) char **__environ = NULL;
+
 extern char **__environ;
 
 size_t __strlen(const char *s) {

autoinstrumentation/injector/src/injector.exports.map

@@ -1,6 +1,7 @@
 {
     global:
         getenv;
+        __environ;
     local:
         *;
 };