x86: make execmem_alloc() cache optional

This let's us evaluate the gains with minimal kernel changes since
the only information we had before this implementation was a
"AB testing" for eBPF claim for about 0.5% gains whatever that means...
and then a student who saw "10% performance improvement" but with
no details spelled out at all.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>