[llvm][cmake] never link llvm-config against llvm dylib

When cross-compiling, in order to make the output of the native and
cross-compiled llvm-config match, one needs to re-pass all cmake flags
relevant to BuildVariables.inc via `CROSS_TOOLCHAIN_FLAGS_NATIVE`. If
`LLVM_LINK_LLVM_DYLIB=ON` is among those, building a full `libLLVM` shared
object is required for the native llvm-config, otherwise `--shared-mode`
will be incorrect and `--link-shared` broken.

To avoid this, we can make llvm-config link statically against the
needed components for simplicity's sake in both the native and cross
case.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D116537