consolidate __xstat abi-compat functions into a single source file

these are mostly intended for use with dynamic linking (although they
can also be used statically with object files compiled against glibc
headers), so having them broken down into separate source files to
optimize for static linking is unlikely to be worth the cost having
more files in the source tree (which contributes to libc.a overhead,
compile time, link time, ar/linker command line size exhaustion, and
so on).
add __xmknod and __xmknodat abi-compat functions

these are put alongside the similar functions for __xstat, etc. in
__xstat.c to avoid bloating the number of source files.
add __sysv_signal abi-compat alias for the signal function

it should be noted that the "real" __sysv_signal, which we do not
implement, is semantically different from signal. references to
__sysv_signal arise in code built against glibc under certain
combinations of feature test macros, and are almost surely
unintentional since the legacy sysv signal behavior has fundamental
race conditions that cannot be worked around and which make it
impossible to use safely.