editors/emacs: fix miscompilation on powerpc64 elfv2

Clang currently miscompiles emacs, resulting in broken builds for dependencies:

> Building for treepy.el-emacs26-0.1.1_3

(cd /wrkdirs/usr/ports/devel/treepy.el/work-full/treepy.el-0.1.1 ; /usr/local/bin/emacs-26.3 -batch -q -f batch-byte-compile treepy.el)

• Signal 6

Use GCC to workaround it.

PR: 243484
Approved by: jrm (maintainer)