[SPARC64]: Fix linkage of enormous kernels.

This was found by make randconfig

If the kernel .text is very large, the .fixup section branches
are too far away to be relocated correctly.

Use "sethi %hi(label), reg; jmpl reg + %lo(label); %g0" sequence
instead of the branch to fix this.

There is another case in switch_to() involving a branch, which
is fixed similarly.

Signed-off-by: David S. Miller <davem@davemloft.net>