bionic-amd64-tf-m-build: Use wget to download ARM Clang

There is an error to download ARM Clang with curl for the time being.
Use wget to download.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I53b4cd7171bfc6a47d34d135864ff90adbf4fe3b
1 file changed