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.