Handle absolute BSS segment location of 'xt'

Handle absolute BSS segment location of 'xt'

When linking an absolute executable, the data
segment and BSS segment can be assigned an
absolute address, or the special value 'x' to
indicate they are contiguous with the prior
segment (text for data, data for BSS). However,
RLN (like MAC) also accepts an optional segment
specifier after the special value 'x' that allows
explicitly specifying which segment the current
one is contiguous with. For data, the only valid
value is 'xt', meaning contiguous with text.
However, when the data segment is specified to
be at an explicit address rather than contiguous
with the text segment, it is valid to specify that
the BSS segment is contiguous with data ('xd') or
with text ('xt'). RLN was accepting these
additional 't' and 'd' values, but ignoring them.

This change causes RLN to correctly place the BSS
segment after the text segment when its location
is specified as 'xt' as long as the data segment
isn't also contiguous with the text segment.