s390: Go closure support
s390: Kill trailing whitespace
s390: Avoid aliasing warnings
s390: Reorganize assembly

Avoid using ffi_prep_args as a callback; do all the work setting
up the frame within ffi_call_int directly.  Save fewer registers
in ffi_closure_SYSV.
s390: Use pc-relative insns in 31-bit mode

It's silly to stick to esa/390 features when the compiler won't.
Detect when brasl and larl are used by the compiler and then use
them in the assembly.
s390: Inline and tidy ffi_prep_args

As per discussion with Ulrich Weigand, document the restrictions
on the code within ffi_call_int as we simultaneously prepare
stack frames for ffi_call_SYSV and the target function.
Merge pull request #158 from rth7680/s390

S390