x86: Always disallow double word suffix with word general register

In 64-bit mode, double word suffix in mnemonic with word general register
is disallowed.  Otherwise, assembler gives a warning:

$ cat /tmp/x.s
	movl	%ax, %bx
	movl	%ds, %ax
	movl	%ax, %cs
$ gcc -c /tmp/x.s
/tmp/x.s: Assembler messages:
/tmp/x.s:1: Error: incorrect register `%bx' used with `l' suffix
/tmp/x.s:2: Error: incorrect register `%ax' used with `l' suffix
/tmp/x.s:3: Error: incorrect register `%ax' used with `l' suffix
$ gcc -c /tmp/x.s -m32
/tmp/x.s: Assembler messages:
/tmp/x.s: Assembler messages:
/tmp/x.s:1: Warning: using `%ebx' instead of `%bx' due to `l' suffix
/tmp/x.s:1: Warning: using `%eax' instead of `%ax' due to `l' suffix
/tmp/x.s:2: Warning: using `%eax' instead of `%ax' due to `l' suffix
/tmp/x.s:3: Warning: using `%eax' instead of `%ax' due to `l' suffix

This patch makes it a hard error in all modes.  Now we get:

$ gcc -c /tmp/x.s -m32
/tmp/x.s: Assembler messages:
/tmp/x.s:1: Error: incorrect register `%bx' used with `l' suffix
/tmp/x.s:2: Error: incorrect register `%ax' used with `l' suffix
/tmp/x.s:3: Error: incorrect register `%ax' used with `l' suffix

	PR gas/25438
	* config/tc-i386.c (check_long_reg): Always disallow double word
	suffix in mnemonic with word general register.
	* testsuite/gas/i386/general.s: Replace word general register
	with double word general register for movl.
	* testsuite/gas/i386/inval.s: Add tests for movl with word general
	register.
	* testsuite/gas/i386/general.l: Updated.
	* testsuite/gas/i386/inval.l: Likewise.