gdb/testsuite: Remove duplicates from gdb.base/funcargs.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/funcargs.exp ...
    DUPLICATE: gdb.base/funcargs.exp: run to call2a

Fix by using proc_with_prefix instead on plain proc to create logical
function blocks.

Tested on x86_64-linux.