[Pal/Linux-SGX] Add distclean target for sgx-driver

The SGX driver is typically updated only very rarely. Therefore,
there is no need to remove the linux-sgx-driver symlink and the
isgx_version.h header in `make clean`. This commit introduces
a new distclean target to remove them.

Jenkinsfiles/Linux-SGX

@@ -111,7 +111,7 @@ pipeline {
                            cd "$(./Scripts/clean-check-test-copy)"
 
                            rm Pal/src/host/Linux-SGX/signer/enclave-key.pem
-                           make -C Pal/src/host/Linux-SGX/sgx-driver clean
+                           make -C Pal/src/host/Linux-SGX/sgx-driver distclean
                            make SGX=1 clean
                            rm LibOS/glibc-*.tar.gz
                            make -C LibOS/shim/test/regression SGX=1 clean

Pal/src/host/Linux-SGX/sgx-driver

@@ -1 +1 @@
-Subproject commit a9b1a30468cffa7102c8b2b1c84c60d03037a702
+Subproject commit e7d1265edc1835290f6eb833166b5e265896d198