Drop the optimisation level for ppc64le cross-compile

The default cross compiler (gcc 9.4.0) for ppc64le on Ubunut 20.04 seems
buggy and causes a seg fault in sslapitest. This doesn't impact any other
CI cross compile platforms and does not seem to impact the gcc 10.3.0 cross
compiler.

We just drop the optimisation level on that platform.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19056)

.github/workflows/cross-compiles.yml

@@ -80,7 +80,10 @@ jobs:
           }, {
             arch: powerpc64le-linux-gnu,
             libs: libc6-dev-ppc64el-cross,
-            target: linux-ppc64le
+            # The default compiler for this platform on Ubuntu 20.04 seems
+            # buggy and causes test failures. Dropping the optimisation level
+            # resolves it.
+            target: -O2 linux-ppc64le
           }, {
             arch: riscv64-linux-gnu,
             libs: libc6-dev-riscv64-cross,