[boringssl] Refactor arch/os specific GN

This CL condenses the CPU and OS detection and pulls it out of the
libcrypto rule.  There's only three supported cases and enumerating
them makes them much clearer.

SEC-44 #comment Using boringssl as an example

Change-Id: I993103699f506163a2de907460d41e36ed9e639a
Tests: Garnet's boringssl_tests
1 file changed