LinkerId: Fix detection of linker tool for XL as nvcc host compiler

When IBM XL is used as the host compiler for nvcc, it generates a
comma-separated link line.  Parse the `exec:` line syntax separately, as
was done in commit b5f20da94d (CMakeParseImplicitLinkInfo supports comma
separated link lines, 2019-08-23, v3.16.0-rc1~181^2).