[ELF] - Allow set CPU string with -plugin-opt=mcpu=<xxx>

[grimar]

Summary

Previously we ignored -plugin-opt=mcpu=<xxx>
and the only way to set CPU string was to pass
-mllvm -mcpu=<xxx>
Though clang may pass it with use of plugin options:
-plugin-opt=mcpu=x86-64
Since we are trying to be compatible in command line
with gold plugin, seems we should support it too.

[ruiu]

LGTM

ELF/Driver.cpp

for (auto *Arg : Args.filtered(OPT_plugin_opt)) {
     else if (S == "save-temps")
       Config->SaveTemps = true;
     else if (S.startswith("O"))
       Config->LTOO = parseInt(S.substr(1), Arg);
     else if (S.startswith("lto-partitions="))
       Config->LTOPartitions = parseInt(S.substr(15), Arg);
     else if (S.startswith("jobs="))
       Config->ThinLTOJobs = parseInt(S.substr(5), Arg);
+    else if (S.startswith("mcpu="))
+      LTOOptions.push_back(Saver.save("-" + S).data());
     else if (!S.startswith("/") && !S.startswith("-fresolution=") &&
-             !S.startswith("-pass-through=") && !S.startswith("mcpu=") &&
-             !S.startswith("thinlto"))
+             !S.startswith("-pass-through=") && !S.startswith("thinlto"))
       LTOOptions.push_back(S.data());
   }
   // Parse and evaluate -mllvm options.
   for (auto *Arg : Args.filtered(OPT_mllvm))
     LTOOptions.push_back(Arg->getValue());
   cl::ParseCommandLineOptions(LTOOptions.size(), LTOOptions.data());
 
   if (Config->LTOO > 3)

test/ELF/lto-plugin-ignore.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -plugin-opt=/foo/bar -plugin-opt=-fresolution=zed \
 # RUN:   -plugin-opt=-pass-through=-lgcc -plugin-opt=-function-sections \
-# RUN:   -plugin-opt=-data-sections -plugin-opt=mcpu=x86-64 \
-# RUN:   -plugin-opt=thinlto -o /dev/null
+# RUN:   -plugin-opt=-data-sections -plugin-opt=thinlto -o /dev/null
 
 # RUN: not ld.lld %t -plugin-opt=-data-sectionxxx \
 # RUN:   -plugin-opt=-function-sectionxxx 2>&1 | FileCheck %s
 # CHECK: Unknown command line argument '-data-sectionxxx'
 # CHECK: Unknown command line argument '-function-sectionxxx'

test/ELF/lto/cpu-string.ll

 ; REQUIRES: x86
 ; RUN: llvm-as %s -o %t.o
 
 ; RUN: ld.lld %t.o -o %t.so -shared
 ; RUN: llvm-objdump -d -section=".text" -no-leading-addr -no-show-raw-insn %t.so | FileCheck %s
+; CHECK: nop{{$}}
 
 ; RUN: ld.lld -mllvm -mcpu=znver1 %t.o -o %t.znver1.so -shared
 ; RUN: llvm-objdump -d -section=".text" -no-leading-addr -no-show-raw-insn %t.znver1.so | FileCheck -check-prefix=ZNVER1 %s
-
-; CHECK: nop{{$}}
-
 ; ZNVER1: nopw
 
+; Check we are able to use -plugin-opt=mcpu=<CPU> to set CPU string.
+; RUN: ld.lld -plugin-opt=mcpu=znver1 %t.o -o %t.znver1.so -shared
+; RUN: llvm-objdump -d -section=".text" -no-leading-addr -no-show-raw-insn %t.znver1.so | FileCheck -check-prefix=ZNVER1 %s
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define void @foo() #0 {
 entry:
   call void asm sideeffect ".p2align        4, 0x90", "~{dirflag},~{fpsr},~{flags}"()
   ret void
 }
 
 attributes #0 = { "no-frame-pointer-elim"="true" }