Use architecture #define's instead of getting atf_arch

This will allow us to run the testcases without tripping asserts
in the atf code.

This is based on the following NetBSD commit:

From: "Julio Merino" <jmmv <at> netbsd.org>
To: gnats-bugs <at> gnats.NetBSD.org
Cc:
Subject: PR/48582 CVS commit: src/tests
Date: Sun, 9 Feb 2014 21:26:07 +0000

 Module Name:	src
 Committed By:	jmmv
 Date:		Sun Feb  9 21:26:07 UTC 2014

 Modified Files:
 	src/tests/include: t_paths.c
 	src/tests/lib/libc/gen: t_fpsetmask.c t_isnan.c t_siginfo.c
 	src/tests/lib/libm: t_ldexp.c t_log.c
 	src/tests/lib/libpthread: t_mutex.c

 Log Message:
 Use compiler builtins instead of atf_arch and atf_machine.

 The atf_arch and atf_machine configuration variables were removed from
 atf-0.19 without me realizing that some tests were querying them directly.

 Instead of reintroducing those variables, just rely on compiler builtins
 as many other tests already do.

 Should fix PR bin/48582.