MIPS: Unify the suffix of compressed vmlinux.bin

MIPS: Unify the suffix of compressed vmlinux.bin

The compressed vmlinux.bin is only a temp file so it's ok to use the same
suffix .z for them (.gz,.lzo,.lzma...) to remove several lines and simpify
the maintenance (no need to add the "suffix_$(xxx) := suffix" line).

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
To: linux-mips <linux-mips@linux-mips.org>
Cc: Alexander Clouter <alex@digriz.org.uk>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Patchwork: https://patchwork.linux-mips.org/patch/1323/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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