[ELF] << >>: make RHS less than 64

The left/right shift linker script operators may trigger UB.
E.g. in linkerscript/end-overflow-check.test, the initial REGION1__PADDED_SR_SHIFT is
uint64_t(-3), cause the following expression to trigger an out-of-range shift in
a ubsan build of lld.

    REGION1__PADDED_SR_SIZE = MAX(1 << REGION1__PADDED_SR_SHIFT, 32);

Protect such UBs by making RHS less than 64.