Add a test to check the initial register state for a new thread

Add a test to check the initial register state for a new thread

This checks that no register starts off with an undefined value
(i.e. a value that comes from uninitialised memory, or that was leaked
from trusted code in another way).  We account for all register
values.

Also fix REGS_SAVER_FUNC() on ARM so that the routine it defines is
correctly aligned when two or more REGS_SAVER_FUNC() are used in the
same source file.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2236
TEST=run_syscall_return_regs_test

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=10061

[bradn, 2012-10-22 17:48:13]

lgtm