fix pread/pwrite syscall calling convention on sh

despite sh not generally using register-pair alignment for 64-bit
syscall arguments, there are arch-specific versions of the syscall
entry points for pread and pwrite which include a dummy argument for
alignment before the 64-bit offset argument.

update syscalls with off_t arguments to handle argument alignment, if needed

the arm syscall abi requires 64-bit arguments to be aligned on an even
register boundary. these new macros facilitate meeting the abi
requirement without imposing significant ugliness on the code.

overhaul pthread cancellation

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

consistency: change all remaining syscalls to use SYS_ rather than __NR_ prefix

syscall overhaul part two - unify public and internal syscall interface

with this patch, the syscallN() functions are no longer needed; a
variadic syscall() macro allows syscalls with anywhere from 0 to 6
arguments to be made with a single macro name. also, manually casting
each non-integer argument with (long) is no longer necessary; the
casts are hidden in the macros.

some source files which depended on being able to define the old macro
SYSCALL_RETURNS_ERRNO have been modified to directly use __syscall()
instead of syscall(). references to SYSCALL_SIGSET_SIZE and SYSCALL_LL
have also been changed.

x86_64 has not been tested, and may need a follow-up commit to fix any
minor bugs/oversights.

finish moving 32-bit-specific junk out of source files.

initial check-in, version 0.5.0