[ELF][RISCV] Make .sdata and .sbss closer

GNU ld's internal linker scripts for RISC-V place .sdata and .sbss close.
This makes GP relaxation more profitable.

While here, when .sbss is present, set `__bss_start` to the start of
.sbss instead of .bss, to match GNU ld.

Note: GNU ld's internal linker scripts have symbol assignments and input
section descriptions which are not relevant for modern systems. We only
add things that make sense.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D145118