s2e.h: : Refactor repetitive x86_64 ifdefs in inline assembly statements.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
s2e.h: fixed incorrect register in s2e_read()

Fixing the style to match the other functions is left for later.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>