selftests: Link libasan statically for tests with -fsanitize=address

When dynamically linking, Address Sanitizer requires its library to be
the first one to be loaded; this is apparently to ensure that every call
to malloc is intercepted. If using LD_PRELOAD, those listed libraries
will be loaded before the libraries listed in the program's ELF and will
therefore violate this requirement, leading to the below failure and
output from ASan.

commit 58e2847a ("selftests: line buffer test program's stdout")
modified the kselftest runner to force line buffering by forcing the
test programs to run through `stdbuf`. It turns out that stdbuf
implements line buffering by injecting a library via LD_PRELOAD.
Therefore selftests that use ASan started failing.

Fix this by statically linking libasan in the affected test programs,
using the `-static-libasan` option. Note this is already the default for
Clang, but not got GCC.

Test output sample for failing case:

  TAP version 13
  1..3
  # timeout set to 300
  # selftests: openat2: openat2_test
  # ==4052==ASan runtime does not come first in initial library list;
  you should either link runtime to your application or manually preload
  it with LD_PRELOAD.
  not ok 1 selftests: openat2: openat2_test # exit=1
  # timeout set to 300
  # selftests: openat2: resolve_test
  # ==4070==ASan runtime does not come first in initial library list;
  you should either link runtime to your application or manually preload
  it with LD_PRELOAD.
  not ok 2 selftests: openat2: resolve_test # exit=1

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Fixes: 58e2847a ("selftests: line buffer test program's stdout")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202309121342.97e2f008-oliver.sang@intel.com