[runtimes] Name stripped install targets consistently

We were previously naming sub-component stripped install targets as
`install-${component}-stripped-${triple}`, whereas everywhere else names
them `install-${component}-${triple}-stripped`. This inconsistency would
cause issues when LLVM_RUNTIME_DISTRIBUTION_COMPONENTS contained a
sub-component (which I'm addding support for next).

Reviewed By: phosek, #libc, #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D138965