gas: deal with the need for relocations from .cfi_{escape,fde_data}

Ignoring return values often isn't a good idea. The Sparc assembler in
particular would report an internal error if an expression with
relocation specifier is used with .cfi_escape, when the same works fine
with .byte. Propagate the relocation indicator up from
do_parse_cons_expression(), and eventually into emit_expr_with_reloc().

dot_cfi_fde_data(), only retaining the expression's X_add_number, would
require further work. Simply report the lack of support there. While
there, also check that what we were dealt is actually a constant.