Make SH ucontext always match current kernels.

Make SH ucontext always match current kernels.

As discussed in the thread starting at
<https://sourceware.org/ml/libc-alpha/2015-06/msg00657.html>, there
are various problems with the sigcontext / mcontext / ucontext
structures on SH.  The soft-float SH4 case in fact does not build at
present, with errors processing
sysdeps/unix/sysv/linux/sh/sh4/ucontext_i.sym with gen-as-const.awk
("error: 'mcontext_t {aka struct <anonymous>}' has no member named
'fpregs'").

Linux 4.8 (commit bbe6c77857c38f4acbdc4fc70399515226d1859a) moved to
always using the same sigcontext structure on SH, with room for
floating-point registers whether or not present on the processor.
This patch makes the glibc header match.

Tested (compilation only) for sh4-linux-gnu hard float, and in
conjunction with other fixes for soft float.

	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h [__SH4__ || __SH4A__]:
	Make code unconditional.
	[!(__SH4__ || __SH4A__)]: Remove conditional code.