sh: make sigcontext definition consistent across fpu/nofpu models

Up until now, the SH version of the sigcontext structure, and thus
mcontext_t/ucontext_t, varied depending on the cpu model the kernel
was built to run on. SH-4 (including SH-4A) and SH-2A used the form
with space for FPU registers, and everything else used a form that
omitted them.

From a userspace perspective, however, the structure layout must be
fixed for a given ABI. Traditionally glibc and uClibc used the form
with space for FPU registers only when __SH4__ (which implies FPU;
__SH4_NOFPU__ is the predefined macro for SH-4 but with no-FPU ABI)
was defined. As a result:

- SH-4 no-FPU programs never matched kernel sigcontext.

- SH-3 programs did not match kernel sigcontext if run on SH-4,
  despite an apparent intent that they be compatible.

- SH-2 and SH-2A programs (using uClibc) did not match kernel
  sigcontext if run on SH-2A.

The mismatch might seem inconsequential because it occurs at the end
of the sigcontext structure, but sigcontext is embedded as uc_mcontext
in ucontext_t, where it is followed by uc_sigmask, an important member
for signal handlers to have access to. In particular, access to
uc_sigmask is necessary for a correct implementation of thread
cancellation.

It would be possible to retain support for both sigcontext ABIs via a
personality mechanism, but since many configurations were already
broken and nobody noticed, and since there are very few if any users
of legacy no-FPU models anymore, I have opted to just remove the
variation and always include space for the FPU registers in
sigcontext. This was proposed and discussed on a thread "SH sigcontext
ABI is broken" cross-posted to linux-sh, libc-alpha, and musl libc
lists in June 2015, and no objections were raised.

Signed-off-by: Rich Felker <dalias@libc.org>

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