Fix ARMv6 detection when CFLAGS modify target CPU architecture

* configure.ac (gcry_cv_cc_arm_arch_is_v6): Use compiler test instead
of preprocessor test.
--

Old test was using C preprocessor to check ARM version macros and missed fact
that using different CFLAGS affect those macros (CFLAGS are not passed to
preprocessor checks).

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>