scripts/objdump-func: Support multiple functions

Allow specifying multiple functions on the cmdline.  Note this removes
the secret EXTRA_ARGS feature.

While at it, spread out the awk to make it more readable.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/0bf5f4f5978660985037b24c6db49b114374eb4d.1681325924.git.jpoimboe@kernel.org