Add missing '.text' in crypto/bn/asm/ppc.pl

Fixes #8495

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8496)

(cherry picked from commit 2864df8f9d3264e19b49a246e272fb513f4c1be3)