[DebugInfo][test][MIPS] Use mtriple in tests

Mips tests are using -march in RUN lines,
this will fail on AIX OS , when we get the mips-ibm-aix triple.

This is caused/exposed recently due to https://reviews.llvm.org/D101194 changed the default getMultiarchTriple in toolchain.

Update the tests to use -mtriple instead to avoid unintended failures.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D101863

Author: Jinsong Ji

llvm/test/DebugInfo/Mips/dsr-fixed-objects.ll

@@ -1,4 +1,4 @@
-; RUN: llc -march=mips -mcpu=mips32r2 -O1 -filetype=obj -relocation-model=pic <%s | \
+; RUN: llc -mtriple=mips -mcpu=mips32r2 -O1 -filetype=obj -relocation-model=pic <%s | \
 ; RUN:    llvm-dwarfdump -v -all - | FileCheck %s
 
 ; void foo(int *);

llvm/test/DebugInfo/Mips/dsr-non-fixed-objects.ll

@@ -1,4 +1,4 @@
-; RUN: llc -march=mips -mcpu=mips32r2 -O0 -filetype=obj -fast-isel=0 <%s | \
+; RUN: llc -mtriple=mips -mcpu=mips32r2 -O0 -filetype=obj -fast-isel=0 <%s | \
 ; RUN:    llvm-dwarfdump -v -all - | FileCheck %s
 
 declare void @llvm.dbg.declare(metadata, metadata, metadata)

llvm/test/DebugInfo/Mips/dwarfdump-tls.ll

@@ -1,13 +1,13 @@
-; RUN: llc -O0 -march=mips -mcpu=mips32r2 -filetype=obj \
+; RUN: llc -O0 -mtriple=mips -mcpu=mips32r2 -filetype=obj \
 ; RUN:     -split-dwarf-file=foo.dwo -o=%t-32.o < %s
 ; RUN: llvm-dwarfdump %t-32.o 2>&1 | FileCheck %s
-; RUN: llc -O0 -march=mips64 -mcpu=mips64r2 -filetype=obj \
+; RUN: llc -O0 -mtriple=mips64 -mcpu=mips64r2 -filetype=obj \
 ; RUN:     -split-dwarf-file=foo.dwo -o=%t-64.o < %s
 ; RUN: llvm-dwarfdump %t-64.o 2>&1 | FileCheck %s
 
-; RUN: llc -O0 -march=mips -mcpu=mips32r2 -filetype=asm \
+; RUN: llc -O0 -mtriple=mips -mcpu=mips32r2 -filetype=asm \
 ; RUN:     -split-dwarf-file=foo.dwo < %s | FileCheck -check-prefix=ASM32 %s
-; RUN: llc -O0 -march=mips64 -mcpu=mips64r2 -filetype=asm \
+; RUN: llc -O0 -mtriple=mips64 -mcpu=mips64r2 -filetype=asm \
 ; RUN:     -split-dwarf-file=foo.dwo < %s | FileCheck -check-prefix=ASM64 %s
 
 @x = thread_local global i32 5, align 4, !dbg !0