tests: use correct m32/mx32 st_mtime_nsec checks in tests/xstatx.c

tests: use correct m32/mx32 st_mtime_nsec checks in tests/xstatx.c

* bootstrap: Add -DMPERS_IS_$(MPERS_NAME) to ARCH_MFLAGS.
* tests/xstatx.c [USE_ASM_STAT && STRUCT_STAT_IS_STAT64 && MPERS_IS_m32]:
Redefine HAVE_STRUCT_STAT_ST_MTIME_NSEC
to HAVE_M32_STRUCT_STAT64_ST_MTIME_NSEC.
[USE_ASM_STAT && STRUCT_STAT_IS_STAT64 && MPERS_IS_mx32]:
Redefine HAVE_STRUCT_STAT_ST_MTIME_NSEC
to HAVE_MX32_STRUCT_STAT64_ST_MTIME_NSEC.
[USE_ASM_STAT && !STRUCT_STAT_IS_STAT64 && MPERS_IS_m32]:
Redefine HAVE_STRUCT_STAT_ST_MTIME_NSEC
to HAVE_M32_STRUCT_STAT_ST_MTIME_NSEC.
[USE_ASM_STAT && !STRUCT_STAT_IS_STAT64 && MPERS_IS_mx32]:
Redefine HAVE_STRUCT_STAT_ST_MTIME_NSEC
to HAVE_MX32_STRUCT_STAT_ST_MTIME_NSEC.

Based on patch by James Clarke <jrtc27@jrtc27.com>.