Update to the 20171126 snapshot of GCC 8.

This requires two temporary patches to allow for building with
clang 3.4.1 (on FreeBSD 10.4) and trimming down an overeager check
that breaks Wine, for example.

Forward port revision 454177 from lang/gcc7: [1]

Make sure what we install is stripped (i.e., debug info is removed).

The straightforward way is setting INSTALL_TARGET to install-strip,
which is supported by the upstream GCC build machinery.

Unfortunately this fails when running as regular user (non-root)
since strip requires write permission to the files in question,
and we install binaries as r-xr-xr-x by default. To work around
that we need to set BINMODE to allow for write access by the user,
something that's common on GNU/Linux (which is why this probably
has not been noticed there). This is not necessary when running
as root.

(A different approach suggested was to set STRIP=true, alas that
leads to many files actually not being stripped. This is due to
GCC using its own script install-sh that in turn uses cp, chmod,
strip,... instead of our own install-* tools in many cases.)

Reported by: Ports QA Framework, miwi, sobomax [1]
Discussed with: tijl, miwi [1]
Differential Revision: https://reviews.freebsd.org/D10357 [1]