kselftest/arm64: bti: force static linking

The "bti" selftests are built with -nostdlib, which apparently
automatically creates a statically linked binary, which is what we want
and need for BTI (to avoid interactions with the dynamic linker).

However this is not true when building a PIE binary, which some
toolchains (Ubuntu) configure as the default.
When compiling btitest with such a toolchain, it will create a
dynamically linked binary, which will probably fail some tests, as the
dynamic linker might not support BTI:
===================
TAP version 13
1..18
not ok 1 nohint_func/call_using_br_x0
not ok 2 nohint_func/call_using_br_x16
not ok 3 nohint_func/call_using_blr
....
===================

To make sure we create static binaries, add an explicit -static on the
linker command line. This forces static linking even if the toolchain
defaults to PIE builds, and fixes btitest runs on BTI enabled machines.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Fixes: 314bcbf0

 ("kselftest: arm64: Add BTI tests")
Link: https://lore.kernel.org/r/20220511172129.2078337-1-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>