vendor.xilinx_{7series,ultrascale}: don't use `write_verilog -decimal`.

vendor.xilinx_{7series,ultrascale}: don't use `write_verilog -decimal`.

In commit 892cff05, `-decimal` was used when writing Verilog for
Vivado targets because it treats (* keep=32'd1 *) and (* keep=1 *)
differently in violation of Verilog LRM. However, it is possible
to avoid that workaround by using (* keep="TRUE" *). Do that,
and remove `-decimal` to avoid special-casing 32-bit constants.

Refs #373.