Move asm machine-dependent files to "src" folder.

Move asm machine-dependent files to "src" folder.

* .gitignore: Add entries from "src" folder; add auto-generated files
by CMake; allow a suffix for "libatomic_ops" folder.
* Makefile.direct (SRCS, mach_dep.o): Move sparc_mach_dep.S,
sparc_sunos4_mach_dep.s, sparc_netbsd_mach_dep.s,
ia64_save_regs_in_stack.s to "src" directory.
* Makefile.dj (SRCS): Ditto.
* configure.ac (machdep): Move sparc_mach_dep.lo,
sparc_netbsd_mach_dep.lo, ia64_save_regs_in_stack.lo to "src" folder.
* PCR-Makefile (mach_dep.o): Remove "if_mach" statement for MIPS.
* PCR-Makefile (if_mach, if_not_there): Remove rules.
* gc.mak (CPP_PROJ): Prefix libatomic_ops folder with "./".