aarch64 (arm64) isn't a supported architecture for exp-sgcheck.

aarch64 (arm64) isn't a supported architecture for exp-sgcheck.

exp-sgcheck/pc_main.c contains:

   #if defined(VGA_arm) || defined(VGA_arm64)
      VG_(printf)("SGCheck doesn't work on ARM yet, sorry.\n");
      VG_(exit)(1);
   #endif

But exp-sgcheck/tests/is_arch_supported checked against uname -m
which returns aarch64 (not arm64). Fix the test check so the
exp-sgcheck tests are skipped instead of producing failures.