riscv: avoid the PIC offset of static percpu data in module beyond 2G limits

The compiler uses the PIC-relative method to access static variables
instead of GOT when the code model is PIC. Therefore, the limitation of
the access range from the instruction to the symbol address is +-2GB.
Under this circumstance, the kernel cannot load a kernel module if this
module has static per-CPU symbols declared by DEFINE_PER_CPU(). The reason
is that kernel relocates the .data..percpu section of the kernel module to
the end of kernel's .data..percpu. Hence, the distance between the per-CPU
symbols and the instruction will exceed the 2GB limits. To solve this
problem, the kernel should place the loaded module in the memory area
[&_end-2G, VMALLOC_END].

Signed-off-by: Vincent Chen <[email protected]>
Suggested-by: Alexandre Ghiti <[email protected]>
Suggested-by: Anup Patel <[email protected]>
Tested-by: Alexandre Ghiti <[email protected]>
Tested-by: Carlos de Paula <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>

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