Avoid using target function attribute in Clang before 3.8
Avoid using target function attribute in Clang before 3.8

We support 3.5 or higher generally, but only Clang 3.8 has the
function attributes. This doesn't affect the build with older Clang
because the makefile still sets file-wide ISA flags.

GCC supports this attribute in all versions we support.

Fixes GH #797

Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,randombit/botan,webmaster128/botan,webmaster128/botan