build: use multiple `install` rather than building up a list

Rather than building up a list to iterate over later, just create multiple
install commands based on the configuration. This makes it easier to see what
is getting installed and allows for the install handling to be centralised. NFC

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365562 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed