Revert "Makefile.uk: Add flag to avoid gcc specifc symbol"

Revert "Makefile.uk: Add flag to avoid gcc specifc symbol"

This reverts commit 4912487a42ec9ab1f7ab7bcca698c50a849eafa8.

__muldc3 is implemented by lib-compiler-rt for clang, and emitting calls
to it is normal. While -ffast-math prevents these calls, it also enables
unsafe optimizations that might result in wrong results from math
functions.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-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: #46