[lld][ELF] Add a corner testcase for elf::getLoongArchPageDelta

If `page(dest) - page(pc)` is 0xfffffffffff000, i.e. page(pc) is next
to page(dest), and lo12(dest) > 0x7ff, correct %pc64_lo12 and %pc64_hi12
should be both -1 (which can be checked with binutils) but they are both
0 on lld. This patch adds such a test showing lld's incorrect behaviour
and following patch will fix this issue.