Add support for SEGMENT_START.

This is a bit of an odd feature. It is normally used in

. = SEGMENT_START(seg, val);

In bfd it evaluates to val or to the value of the corresponding
-T<seg>-segment. Note that the -T<seg>-segment in bfd doesn't actually
change the segment address, just the value this evaluates too,
including in the default linker script.

In gold the -T<seg>-segment options do change the segment address and
seeing this expressions in linker scripts disables the options.

For new this just always evaluates the expression to val.

llvm-svn: 277014