devel/llvm13: fix building software with -fstack-protector-strong

Apply https://reviews.llvm.org/D109090
Fixes immediate segfault when executing binaries built by the port's compiler with -fstack-protector-strong.

Submitted by: adalava

(cherry picked from commit 531a51cc368cf2f53fd21923532964ba96965474)

devel/llvm13: revert to the older powerpc64 fix

The newer one apparently causes a regression on amd64 freestanding binaries.

While here, fix some whitespace issues in Makefile.

Submitted by: adalava

(cherry picked from commit d326758c32dff2938c1da97c0df5926612c3efa2)