If gcc is 4.8+ or clang is 6.0+, and the architecture implements either
PTRACE_GETREGS or PTRACE_GETREGSET, enable compiler -Werror option
by default.

* configure.ac: Call st_WARN_CFLAGS and AX_PROG_CC_FOR_BUILD after
architecture checks.
(arch_enable_Werror): New variable, defined to "yes" on all architectures
except alpha, bfin, hppa, microblaze, sh, sh64, and xtensa.
* m4/st_warn_cflags.m4 (st_cv_cc_enable_Werror, st_cv_enable_Werror):
Compute new cached variables.
(AC_ARG_ENABLE(gcc-Werror)): Default to $st_cv_enable_Werror
if no option was given, do not add -Werror to WARN_CFLAGS.
[enable_gcc_Werror == yes]: Add -Werror to WARN_CFLAGS.
* m4/ax_prog_cc_for_build.m4 (AX_PROG_CC_FOR_BUILD): Redirect
st_cv_enable_Werror and st_cv_cc_enable_Werror using pushdef/popdef.
* ci/run-build-and-tests.sh (DISTCHECK_CONFIGURE_FLAGS): Add
--enable-gcc-Werror unconditionally.