Some target toolchains use more than just a default --sysroot, but also include a default --dyld-prefix and an implicitly-added -rpath.
For example, using a vanilla build of Clang with the IBM Advance Toolchain for Linux on Power would require specifying --sysroot=/opt/at12.0 --dyld-prefix=/opt/at12.0 -rpath /opt/at12.0/lib64. The GCC compiler provided with the Advance Toolchain is preconfigured such that adding such options is not necessary. This patch adds the configuration hooks that would allow Clang to be preconfigured similarly.
Various tests not compatible with builds that set a non-empty DEFAULT_DYLD_PREFIX (because they are sensitive to the dynamic linker path) are updated with --dyld-prefix="" in a manner analogous to what was done in D66834 with --sysroot="" in relation to DEFAULT_SYSROOT.
Note: The DEFAULT_RPATH behaviour has only been implemented for "GNU" toolchains.