bn: Fix .size directive

This requires the text address.

Fixes #15923

Signed-off-by: Martin Schwenke <martin@meltin.net>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15971)

crypto/bn/asm/ppc64-mont-fixed.pl

@@ -322,7 +322,7 @@ ___
 	$self->add_code(<<___);
 	li		r3,1
 	blr
-.size ${fname},.-${fname}
+.size .${fname},.-.${fname}
 ___
 
 }