[Clang] Ignore CLANG_DEFAULT_LINKER for custom-linker toolchains

Before, the CLANG_DEFAULT_LINKER cmake option was a global override for
the linker that shall be used on all toolchains.  The linker binary
specified that way may not be available on toolchains with custom
linkers. Eg, the only linker for VE is named 'nld' - any other linker
invalidates the toolchain.

This patch removes the hard override and instead lets the generic
toolchain implementation default to CLANG_DEFAULT_LINKER.  Toolchains
can now deviate with a custom linker name or deliberatly default to
CLANG_DEFAULT_LINKER.

Reviewed By: MaskRay, phosek

Differential Revision: https://reviews.llvm.org/D115045