A couple of FP-related fixes (prompted by Adam having trouble

A couple of FP-related fixes (prompted by Adam having trouble
running SPEC FP codes).

arch/alpha/isa_desc:
    Don't warn about non-standard trapping modes more than
    once per static instruction.  (Had the flag to suppress
    these but forgot to check it!)
build/SConstruct:
    Add USE_SSE2 option to enable compiling w/SSE2 (important
    for getting IEEE-compliant FP on x86).

--HG--
extra : convert_revision : eac69efb28cce7b48035480d8b7cb004782969f4