MIPS: Fix build warning about "PTR_STR" redefinition

PTR_STR is redefined when CONFIG_TEST_PRINTF is set. This causes the
following build warning:

  CC      lib/test_printf.o
lib/test_printf.c:214:0: warning: "PTR_STR" redefined
 #define PTR_STR "ffff0123456789ab"
 ^
In file included from ./arch/mips/include/asm/dsemul.h:11:0,
                 from ./arch/mips/include/asm/processor.h:22,
                 from ./arch/mips/include/asm/thread_info.h:16,
                 from ./include/linux/thread_info.h:38,
                 from ./include/asm-generic/preempt.h:5,
                 from ./arch/mips/include/generated/asm/preempt.h:1,
                 from ./include/linux/preempt.h:78,
                 from ./include/linux/spinlock.h:51,
                 from ./include/linux/seqlock.h:36,
                 from ./include/linux/time.h:6,
                 from ./include/linux/stat.h:19,
                 from ./include/linux/module.h:13,
                 from lib/test_printf.c:10:
./arch/mips/include/asm/inst.h:20:0: note: this is the location of the previous definition
 #define PTR_STR  ".dword"
 ^

Instead of renaming PTR_STR we move the unaligned macros to a new file,
which is only included inside MIPS code. This way we can safely include
asm.h and can use STR(PTR) again.

Fixes: e701656e ("MIPS: inst.h: Stop including asm.h to avoid various build failures")
Cc: Maciej W. Rozycki" <macro@linux-mips.org>
Reported-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Co-developed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>