Fix two bugs exposed when trying to boot HEAD on sparc with NCG and -O:

1.  StScratchWords on sparc were in the wrong place; they were
    immediately above %fp and should have been immediately below.
    Fixed.  Also removed a suspicious-looking "+1" in the x86
    version of same.

2.  (Potentially affects all platforms): Lift strings out from
    top-level literal data, and place them at the end of the block.
    The motivating example (bug) was:

     Stix:
        (DataSegment)
        Bogon.ping_closure :
        (Data P_ Addr.A#_static_info)
        (Data StgAddr (Str `alalal'))
        (Data P_ (0))
     results in:
        .data
                .align 8
        .global Bogon_ping_closure
        Bogon_ping_closure:
                .long   Addr_Azh_static_info
                .long   .Ln1a8
        .Ln1a8:
                .byte   0x61
                .byte   0x6C
                .byte   0x61
                .byte   0x6C
                .byte   0x61
                .byte   0x6C
                .byte   0x00
                .long   0
   ie, the Str is planted in-line, when what we really meant was to place
   a _reference_ to the string there.  This is Way Wrong (tm).  Fixed.