Fix gdb.base/share-env-with-gdbserver.exp with Clang

Fix gdb.base/share-env-with-gdbserver.exp with Clang

The testcase has GDB call my_getenv in the inferior, and that fails
with Clang, because Clang optimizes out my_getenv completely, since it
isn't called anywhere (in the program).

This commit fixes it.

gdb/testsuite/ChangeLog:

	* gdb.base/share-env-with-gdbserver.c (main): Call my_getenv
	instead of getenv.