Bug 21053: sh: Reduce namespace pollution from sys/ucontext.h

Bug 21053: sh: Reduce namespace pollution from sys/ucontext.h

The problem is basically that sys/ucontext.h is defining R0..R15
which happens to conflict with some packages like Firefox when
trying to build on SH.

The very same problem existed on arm back then [1] and it was fixed by
renaming R0..R15 to REG_R0..REG_R15.  This patch imploy a similar
strategy for SH.

Checked on sh4-linux-gnu with run-built-tests=no and I also got reports
that it fixes Firefox build on Debian sh4.

	* sysdeps/unix/sysv/linux/sh/sh3/ucontext_i.sym: Use new REG_R*
	constants instead of the old R* ones.
	* sysdeps/unix/sysv/linux/sh/sh4/ucontext_i.sym: Likewise.
	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h (NGPREG): Rename...
	(NGREG): ... to this, to fit in with other architectures.
	(gpregset_t): Use new NGREG macro.
	[__USE_GNU]: Remove condition; all architectures other than tile
	are unconditional.
	(R*): Rename to REG_R*.