MIPS: inst.h: Stop including asm.h to avoid various build failures

Commit d339cd02 ("MIPS: Move unaligned load/store helpers to
inst.h") causes a lot of build failures because macros in asm.h conflict
with various subsystems. Some of these conflictions has been fixed (such
as LONG, PANIC and PRINT) by adjusting asm.h, but some of them is nearly
impossible to fix (such as PTR and END). The only reason of including
asm.h in inst.h is that we need the PTR macro which is used by unaligned
load/store helpers. So in this patch we define a new PTR_STR macro and
use it to replace STR(PTR), then we can stop including asm.h to avoid
various build failures.

Fixes: d339cd02 ("MIPS: Move unaligned load/store helpers to inst.h")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>