[flang] Allow different linker name (#77849)

The linker name is different if the `lld` project is enabled
`-DLLVM_ENABLE_PROJECTS="...;lld"`.

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Co-authored-by: Leandro Lupori <leandro.lupori@gmail.com>

Author: kkwli

flang/test/Driver/isysroot.f90

@@ -6,7 +6,7 @@
 ! RUN: %flang -### --target=aarch64-linux-gnu -isysroot /path/to/sysroot \
 ! RUN:        %s 2>&1 | FileCheck %s --check-prefix=CHECK-LINUX
 
-! CHECK-DARWIN: "{{.*[\/]}}ld" {{.*}}"-syslibroot" "/path/to/sysroot"
+! CHECK-DARWIN: "{{.*}}ld{{(64)?(\.lld)?}}" {{.*}}"-syslibroot" "/path/to/sysroot"
 ! Unused on Linux.
 ! CHECK-LINUX: warning: argument unused during compilation: '-isysroot /path/to/sysroot'
 ! CHECK-LINUX-NOT: /path/to/sysroot