selftests/sgx: Fix a benign linker warning

The enclave binary (test_encl.elf) is built with only three sections (tcs,
text, and data) as controlled by its custom linker script.

If gcc is built with "--enable-linker-build-id" (this appears to be a
common configuration even if it is by default off) then gcc
will pass "--build-id" to the linker that will prompt it (the linker) to
write unique bits identifying the linked file to a ".note.gnu.build-id"
section.

The section ".note.gnu.build-id" does not exist in the test enclave
resulting in the following warning emitted by the linker:

/usr/bin/ld: warning: .note.gnu.build-id section discarded, --build-id
ignored

The test enclave does not use the build id within the binary so fix the
warning by passing a build id of "none" to the linker that will disable the
setting from any earlier "--build-id" options and thus disable the attempt
to write the build id to a ".note.gnu.build-id" section that does not
exist.

Link: https://lore.kernel.org/linux-sgx/20191017030340.18301-2-sean.j.christopherson@intel.com/Suggested-by: Cedric Xing <cedric.xing@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/ca0f8a81fc1e78af9bdbc6a88e0f9c37d82e53f2.1636997631.git.reinette.chatre@intel.com

tools/testing/selftests/sgx/Makefile

@@ -45,7 +45,7 @@ $(OUTPUT)/sign_key.o: sign_key.S
 	$(CC) $(HOST_CFLAGS) -c $< -o $@
 
 $(OUTPUT)/test_encl.elf: test_encl.lds test_encl.c test_encl_bootstrap.S
-	$(CC) $(ENCL_CFLAGS) -T $^ -o $@
+	$(CC) $(ENCL_CFLAGS) -T $^ -o $@ -Wl,--build-id=none
 
 EXTRA_CLEAN := \
 	$(OUTPUT)/test_encl.elf \