app-portage/prefix-toolkit: stack: set AT_SYS_M4DIR, run gcc-config

In stacked Prefix, need to set AT_SYS_M4DIR so eautoreconf is able to
find the m4 files from stacked Prefix.
As prefix-toolkit does wrap the compiler found in base prefix to get the
additional system paths in, need to perform gcc-config at pkg_postinst.

Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Michael Haubenwallner <[email protected]>