add ASM_OUTPUT_MAX_SKIP_ALIGN to i386.h

add ASM_OUTPUT_MAX_SKIP_ALIGN to i386.h

Several i386 align tests expect p2align to be used, but not all
configurations define ASM_OUTPUT_MAX_SKIP_ALIGN, even when
HAVE_GAS_MAX_SKIP_P2ALIGN.

i386.h had an equivalent ASM_OUTPUT_MAX_SKIP_PAD.  I've renamed it and
its uses to the documented _ALIGN spelling, and dropped all redundant
defines elsewhere in gcc/config/i386/.

for  gcc/ChangeLog

	* config/i386/i386.h (ASM_OUTPUT_MAX_SKIP_PAD): Rename to...
	(ASM_OUTPUT_MAX_SKIP_ALIGN): ... this.  Enclose in do/while(0).
	* config/i386/i386.c: Adjust.
	* config/i386/i386.md: Adjust.
	* config/i386/darwin.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Drop.
	* config/i386/dragonfly.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/freebsd.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/gas.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/gnu-user.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/iamcu.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/lynx.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/netbsd-elf.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/openbsdelf.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	* config/i386/x86-64.h (ASM_OUTPUT_MAX_SKIP_ALIGN): Likewise.
	(ASM_OUTPUT_MAX_SKIP_PAD): Likewise.