Improve PPC target function attribute checks

* configure.ac (gcry_cv_gcc_attribute_ppc_target)
(gcry_cv_clang_attribute_ppc_target): Add 'always_inline'
function to test.

With some CFLAG combinations, target attribute fails to work with
always_inline functions. Patch adds detection for such configuration
and disables target attribute use in such case (and suffer less
optimal code generation).

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