gcc: quiet Wattribute for no_sanitize("address")

[rlibby]

Summary

This is an unfortunate instances where the __has_attribute check does
not function usefully. Gcc does have the attribute, but for gcc it only
applies to functions, not variables, and trying to apply it to a
variable generates Wattribute. So far we only apply the attribute to
variables. Only enable the attribute for clang, for now.

Test Plan

make buildkernel
make CROSS_TOOLCHAIN=amd64-gcc9 buildkernel

[rang_acm.org]

Works for now. If I have time over the break I’ll take a look at whether gcc could easily be extended to apply this attribute to variables where it makes sense.