Makefile: adopt --std=gnu11 for HOSTCFLAGS on Linux

Makefile: adopt --std=gnu11 for HOSTCFLAGS on Linux

Following the conversion of the SPDX license tags, a number of files
compiled with -pedantic now generate warnings similar to the following
for using C99-style '//' comments in ISO C90 code:

  tools/gen_eth_addr.c:1:1: warning: C++ style comments are not allowed in ISO C90
   // SPDX-License-Identifier: GPL-2.0+
   ^

The SPDX comment-style change means that these files have adopted C99,
so need to change the language-standard to --std=gnu99 or --std=gnu11
to let the compiler know this.

As we now require GCC 6 or newer for the cross-compiler, the project has
implicitly moved the project to GNU11: let older GCC versions on various
Linux distros know to treat our host tools as GNU11 as well.

References: commit b7f874f3a014 ("SPDX: Convert all of our single license tags to Linux Kernel style")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Tom Rini <trini@konsulko.com>