4.2+ stable inclusion request: ARCv2: save r30 on kernel entry as gcc uses it for code-gen

[Vineet Gupta]

Hi,

Can we please backport upstream ecd43afdbe72017aefe48080631eb625e177ef4d ("ARCv2:
save r30 on kernel entry as gcc uses it for code-gen") to 4.2 to 4.9 kernels.

While the issue cites gcc, it is general deficiency of kernel as userspace
programs can modify r30 register in hand/inline assembly and those will not be
preserved without this change.

Thx,
-Vineet