Makefile.uk: Add flag to avoid gcc specifc symbol

Makefile.uk: Add flag to avoid gcc specifc symbol

When building apps with `clang` for `x86`, an asm specific symbol,
`__muldc3`, appears, which results in a undefined reference error, at
link time.

This commit adds the `-ffast-math` flag, when `clang` is used in order
to fix this issue.

Signed-off-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #38