gcc: don't pass --enable-standard-branch-protection

By changing the default code generation of GCC we're inadvertently
breaking the GCC test suite, which has ~120K+ more failures when run for
aarch64 compared to x86-64.

This was because the generated code fragments included the BTI
instructions, which the test case wasn't expecting.  We can't tell the
tests globally to run without branch protection, as that will break the
tests which also turn it on.

Remove the enabling of branch protection by standard in GCC, we'll
enable it in the tune files instead.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bb4b9017)
Signed-off-by: Steve Sakoman <steve@sakoman.com>