misc: Switch from ISO C11 to GNU C11

We use a number of compiler extensions already, and the strict rules
required for compliance with ISO C11 can be prohitive for embedded
development.

This commit moves our C variant from ISO C11 to GNU C11, which is
compatible with all the toolchains we currently support.

Change-Id: I58ce6479c4f8ca9039d957d08aadd3ebff4ac8fc
Signed-off-by: Chris Kay <chris.kay@arm.com>