Revert "Move asm machine-dependent files to 'src' folder" (partly)

Revert "Move asm machine-dependent files to 'src' folder" (partly)

Asm files moved back to base folder to avoid build issues (reported
for NetBSD and Solaris on Sparc).  Alternative way is to adjust
configure.ac and Makefile.am properly but it requires more efforts
(including testing).

* .gitignore: Remove "src" from comment.
* Makefile.am (EXTRA_libgc_la_SOURCES): Remove "src/" prefix.
* Makefile.direct (SRCS, mach_dep.o): Likewise.
* src/ia64_save_regs_in_stack.s: Move to base folder.
* src/sparc_mach_dep.S: Likewise.
* src/sparc_netbsd_mach_dep.s: Likewise.
* src/sparc_sunos4_mach_dep.s: Likewise.