When `awk` is gawk 5.0.1, the cfi awk scripts print a warning
due to unnecessarily escaping '@' character (see attached patch).

Warning message example:

awk: ./tools/add-cfi.x86_64.awk:79: warning: regexp escape sequence `\@' is not a known regexp operator

Trivial patch addressing this is attached.

Unsure which awk implementations are relevant,
tested with gawk 4.2.1, gawk 5.0.1, and mawk 1.3.4.
Also tested with busybox 1.31.1, didn't check config options used (if that matters).

For all, `.cfi_startproc` seems to be inserted appropriately on example used for testing
(src/math/x86_64/lrintf.s) and no warnings were printed.

Minor issue, but seems might as well fix if there aren't awk's requiring escaping '@'.

Thanks!

~Will