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- getegid, geteuid, getgid, and getuid now return EPERM
  instead of raising SIGSYS. This simplifies the test code.
- Define a macro for sandbox tests for syscalls which return
  EPERM to reduce the size of not interesting test cases.
- Pass five zeros after syscall number to syscall() so that
  tests will not be flaky due to uninitialized values on stack.

BUG=359285
TEST=nacl_loader_unittests, trybots

Review URL: https://codereview.chromium.org/248633004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266317 0039d316-1c4b-4281-b951-d872f2087c98