[Bug target/65181] Support for alloca in nvptx

[tschwinge at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65181

--- Comment #5 from Thomas Schwinge <tschwinge at gcc dot gnu.org> ---
While reviewing other things, I found that "'alloca' is a preview feature in
PTX ISA version 7.3", and "requires 'sm_52' or higher".  That got published
2021-05, so it'll be a while until we can reliably use it, but we're getting
there...  ;-)