[libc] Do not find system binaries for offloading tools

    We use `find_program` to identify a few programs we use for offloading.
    Namely, `clang-offload-packger`, `amdgpu-arch`, and `nvptx-arch`.
    Currently the logic allows these to bind to any tool matching this name,
    so it will find it on the system. This meant that if the installation
    was deleted or it found a broken binary the compilation would fail. We
    should only pull these from the current LLVM binary directory.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158203

Author: jhuber6

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -30,12 +30,12 @@ endif()
 
 # Identify any locally installed AMD GPUs on the system using 'amdgpu-arch'.
 find_program(LIBC_AMDGPU_ARCH
-             NAMES amdgpu-arch
+             NAMES amdgpu-arch NO_DEFAULT_PATH
              PATHS ${LLVM_BINARY_DIR}/bin /opt/rocm/llvm/bin/)
 
 # Identify any locally installed NVIDIA GPUs on the system using 'nvptx-arch'.
 find_program(LIBC_NVPTX_ARCH
-             NAMES nvptx-arch
+             NAMES nvptx-arch NO_DEFAULT_PATH
              PATHS ${LLVM_BINARY_DIR}/bin)
 
 # Get the list of all natively supported GPU architectures.
@@ -64,7 +64,7 @@ message(STATUS "Building libc for the following GPU architecture(s): "
 
 # Identify the program used to package multiple images into a single binary.
 find_program(LIBC_CLANG_OFFLOAD_PACKAGER
-             NAMES clang-offload-packager
+             NAMES clang-offload-packager NO_DEFAULT_PATH
              PATHS ${LLVM_BINARY_DIR}/bin)
 if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
   message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "