runtime: Add __sparc__ case for SETCONTEXT_CLOBBERS_TLS.

The glibc setcontext incorrectly modifies %g7 on SPARC.

From Richard Henderson.

From-SVN: r219778

libgo/runtime/proc.c

@@ -126,6 +126,30 @@ fixcontext(ucontext_t* c)
 	c->uc_mcontext._mc_tlsbase = tlsbase;
 }
 
+# elif defined(__sparc__)
+
+static inline void
+initcontext(void)
+{
+}
+
+static inline void
+fixcontext(ucontext_t *c)
+{
+	/* ??? Using 
+	     register unsigned long thread __asm__("%g7");
+	     c->uc_mcontext.gregs[REG_G7] = thread;
+	   results in
+	     error: variable ‘thread’ might be clobbered by \
+		‘longjmp’ or ‘vfork’ [-Werror=clobbered]
+	   which ought to be false, as %g7 is a fixed register.  */
+
+	if (sizeof (c->uc_mcontext.gregs[REG_G7]) == 8)
+		asm ("stx %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
+	else
+		asm ("st %%g7, %0" : "=m"(c->uc_mcontext.gregs[REG_G7]));
+}
+
 # else
 
 #  error unknown case for SETCONTEXT_CLOBBERS_TLS