Fix various assembler testsuite failures for the Z80 target.

Fix various assembler testsuite failures for the Z80 target.

	PR 25377
gas	* config/tc-z80.c: Add support for half precision, single
	precision and double precision floating point values.
	* config/tc-z80.h b/gas/config/tc-z80.h: Disable string escapes.
	* doc/as.texi: Add new z80 command line options.
	* doc/c-z80.texi: Document new z80 command line options.
	* testsuite/gas/z80/ez80_pref_dis.s: New test.
	* testsuite/gas/z80/ez80_pref_dis.d: New test driver.
	* testsuite/gas/z80/z80.exp: Run the new test.
	* testsuite/gas/z80/fp_math48.d: Use correct command line option.
	* testsuite/gas/z80/fp_zeda32.d: Likewise.
	* testsuite/gas/z80/strings.d: Update expected output.

opcodes	* z80-dis.c (suffix): Use .db instruction to generate double
	prefix.