Switch compiler to clang

Clang now is performant enough to use instead of GCC. We can later
switch on ftrapv to better deal with potential problems. Clang was
having problems with BN multiply operations, but it appears to not
affect RSA speeds anymore.

gcc:
openssl speed -elapsed rsa
                  sign    verify    sign/s verify/s
rsa  512 bits 0.000759s 0.000073s   1317.5  13621.0
rsa 1024 bits 0.004109s 0.000234s    243.4   4276.1
rsa 2048 bits 0.027652s 0.000842s     36.2   1188.3
rsa 4096 bits 0.198824s 0.003204s      5.0    312.1

clang:
openssl speed -elapsed rsa
                  sign    verify    sign/s verify/s
rsa  512 bits 0.000784s 0.000074s   1274.9  13522.6
rsa 1024 bits 0.004132s 0.000235s    242.0   4256.9
rsa 2048 bits 0.027680s 0.000851s     36.1   1175.5
rsa 4096 bits 0.198824s 0.003245s      5.0    308.2

Change-Id: Iadad4739cf925a552158463047bdf1e19f9d0e83