[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.

(cherry picked from commit e752b58)

Author: SixWeining

lld/test/ELF/loongarch-pc-aligned.s

@@ -260,6 +260,19 @@
 # EXTREME15-NEXT: lu32i.d   $t0, -349526
 # EXTREME15-NEXT: lu52i.d   $t0, $t0, -1093
 
+## FIXME: Correct %pc64_lo20 should be 0xfffff (-1) and %pc64_hi12 should be 0xfff (-1), but current values are:
+## page delta = 0x0000000000000000, page offset = 0x888
+## %pc_lo12   = 0x888 = -1912
+## %pc_hi20   = 0x00000 = 0
+## %pc64_lo20 = 0x00000 = 0
+## %pc64_hi12 = 0x00000 = 0
+# RUN: ld.lld %t/extreme.o --section-start=.rodata=0x0000000012344888 --section-start=.text=0x0000000012345678 -o %t/extreme16
+# RUN: llvm-objdump -d --no-show-raw-insn %t/extreme16 | FileCheck %s --check-prefix=EXTREME16
+# EXTREME16:      addi.d $t0, $zero, -1912
+# EXTREME16-NEXT: pcalau12i $t1, 0
+# EXTREME16-NEXT: lu32i.d   $t0, 0
+# EXTREME16-NEXT: lu52i.d   $t0, $t0, 0
+
 #--- a.s
 .rodata
 x: