Add ElfUtils and BinUtils Download URL Overrides (#358)

* Add CMake CACHE Variable ElfUtils_DOWNLOAD_URL

Used to override the default URL to download ElfUtils from.
Useful for internal builds

Also, include a mirror to fallback to if the override URL fails.

* Update timemory submodule

Updating to include the BINUTIL_DOWNLOAD_URL override cmake
variable.

---------

Signed-off-by: David Galiffi <David.Galiffi@amd.com>

cmake/ElfUtils.cmake

@@ -102,7 +102,8 @@ set(_eu_build_byproducts "${_eu_root}/lib/libdw${CMAKE_STATIC_LIBRARY_SUFFIX}"
 externalproject_add(
     omnitrace-elfutils-build
     PREFIX ${PROJECT_BINARY_DIR}/external/elfutils
-    URL https://sourceware.org/elfutils/ftp/${ElfUtils_DOWNLOAD_VERSION}/elfutils-${ElfUtils_DOWNLOAD_VERSION}.tar.bz2
+    URL ${ElfUtils_DOWNLOAD_URL}
+        "https://sourceware.org/elfutils/ftp/${ElfUtils_DOWNLOAD_VERSION}/elfutils-${ElfUtils_DOWNLOAD_VERSION}.tar.bz2"
     BUILD_IN_SOURCE 1
     CONFIGURE_COMMAND
         ${CMAKE_COMMAND} -E env CC=${ElfUtils_C_COMPILER}