This change copies the existing amd64_cloudabi64.c to amd64_cloudabi32.c
and reimplements the functions for fetching system call arguments and
return values to use the same scheme as used by the vDSO that is used
when running cloudabi32 executables.
As arguments are automatically padded to 64-bit words by the vDSO in
userspace, we can copy the arguments directly into the array used by
truss(8) internally.
This change also has some changes for libsysdecode. These will be
committed separately. For libsysdecode, we add a new ABI type,
SYSDECODE_ABI_CLOUDABI32. We also remove all of the #ifdef bits, as
these are not needed. The cloudabi32 and cloudabi64 system call tables
and headers can be built on any FreeBSD architecture without any issues.