x86-64: fix a problem exposed by negative offsets in vector tables

static relative offsets (eg .long l1-l2) are restricted to 32 bits on
x86-64 due to lack of support in the linker.  The codegen, NCG and
runtime work around this, using 32-bit offsets instead of 64.
However, we were missing a workaround for vector tables, and it
happened to work by accident because the offsets were always positive
and resolved by the assembler.  The bug was exposed by using the NCG
to compile the RTS, where the offsets became negative, again by
accident.