PDP11: Use `const_double_zero' to express double zero constant

PDP11: Use `const_double_zero' to express double zero constant

We do not define a comparison operation between floating-point and
integer data, including integer zero constant.  Consequently the RTL
instruction stream presented to the post-reload comparison elimination
pass will include, where applicable, floating-point comparison insns
against `const_double:DF 0.0 [0x0.0p+0]' rather than `const_int 0 [0]',
meaning that the latter expression will not match when used in machine
description.

Use `const_double_zero' then for the relevant patterns to match the
intended RTL instructions.

	gcc/
	* config/pdp11/pdp11.md (fcc_cc, fcc_ccnz): Use
	`const_double_zero' to express double zero constant.