Use llvm::sys::Process::GetEnv() instead of ::getenv().

Process::GetEnv() uses GetEnvironmentVariableW, which is a Windows API
to get an environment variable and is preferable over getenv().

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190431 91177308-0d34-0410-b5e6-96231b3b80d8