ELF/AArch64: Fix R_AARCH64_LDST16_ABS_LO12_NC mask

The relocation R_AARCH64_LDST16_ABS_LO12_NC should set a ld/st
immediate value to bits [11:1] not [11:2].  This patches fixes it
and adds a testcase for regression.

With this fix all the faulty tests on test-suite (clavm, lencod,
and trimaran) pass.

llvm-svn: 288670