[SYCL][E2E] Initialize arch_flag to an empty string by default

[ayylol]

In an environment with no devices this variable is never set, causing a confusing error when we try to define the clangxx expansion. Here we set it to an empty string by default so that instead of exiting with a python error we report all tests as unsupported (due to having no devices).

This same issue is why arch_flag was set in build-only mode, so I removed the line that did that too.

[sarnex]

lgtm, btw what was the confusing error?

[ayylol]

lgtm, btw what was the confusing error?

an error due to arch_flag not being defined

llvm-lit: /home/llvm/llvm/utils/lit/lit/TestingConfig.py:156: fatal: unable to parse config file '/home/llvm/sycl/test-e2e/lit.cfg.py', traceback: Traceback (most recent call last):
  File "/home/llvm/llvm/utils/lit/lit/TestingConfig.py", line 144, in load_from_path
    exec(compile(data, path, "exec"), cfg_globals, None)
  File "/home/llvm/sycl/test-e2e/lit.cfg.py", line 819, in <module>
    " " + config.dpcpp_compiler + " " + config.cxx_flags + " " + arch_flag,
NameError: name 'arch_flag' is not defined

Very niche case, since it can only happen if we have no devices, which is why it needed to be set explicitly for build-only

[sarnex]

cool thx