[lld][ELF] Add getBitcodeMachineKind test for LoongArch (#71931)

Test that getBitcodeMachineKind can get right e_machine for
Triple::loongarch64 and Triple::loongarch32 during LTO.

Author: GongMengyao

lld/test/ELF/lto/loongarch.ll

@@ -0,0 +1,32 @@
+; REQUIRES: loongarch
+;; Test we can infer the e_machine value EM_LOONGARCH from a bitcode file.
+
+; RUN: split-file %s %t
+; RUN: llvm-as %t/32.ll -o %t/32.o
+; RUN: ld.lld %t/32.o -o %t/32
+; RUN: llvm-readobj -h %t/32 | FileCheck %s --check-prefixes=CHECK,LA32
+
+; RUN: llvm-as %t/64.ll -o %t/64.o
+; RUN: ld.lld %t/64.o -o %t/64
+; RUN: llvm-readobj -h %t/64 | FileCheck %s --check-prefixes=CHECK,LA64
+
+; LA32:    Class: 32-bit
+; LA64:    Class: 64-bit
+; CHECK:   DataEncoding: LittleEndian
+; CHECK: Machine: EM_LOONGARCH
+
+;--- 32.ll
+target datalayout = "e-m:e-p:32:32-i64:64-n32-S128"
+target triple = "loongarch32-unknown-elf"
+
+define void @_start() {
+  ret void
+}
+
+;--- 64.ll
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n64-S128"
+target triple = "loongarch64-unknown-elf"
+
+define void @_start() {
+  ret void
+}