Use an absolute path to C/C++ compilers for better compatibility

[brians-neptune]

toolchains_llvm only triggers its logic to locate files relative to the compiler wrapper when called with an absolute path, for example. Otherwise it assumes it's being called from the bazel execroot, which is not true in this case.

toolchains_llvm already has similar logic as wheel_builder for mapping paths, which could be used by just calling it with an absolute path instead of using wheel_builder's wrapper, but I think it's easier to use the wheel_builder version instead of trying to detect whether the toolchain has this logic.