the weak version of __syscall_cp_c was using a tail call to __syscall
to avoid duplicating the 6-argument syscall code inline in small
static-linked programs, but now that __syscall no longer exists, the
inline expansion is no longer duplication.

the syscall.h machinery suppported up to 7 syscall arguments, only via
an external __syscall function, but we presently have no syscall call
points that actually make use of that many, and the kernel only
defines 7-argument calling conventions for arm, powerpc (32-bit), and
sh. if it turns out we need them in the future, they can easily be
added.

no syscalls actually use that many arguments; the issue is that some
syscalls with 64-bit arguments have them ordered badly so that
breaking them into aligned 32-bit half-arguments wastes slots with
padding, and a 7th slot is needed for the last argument.

this code was using $10 to save the syscall number, but $10 is not
necessarily preserved by the kernel across syscalls. only mattered for
syscalls that got interrupted by a signal and restarted. as far as i
can tell, $25 is preserved by the kernel across syscalls.

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.