arch/x86: Add compiler flags for auto host CPU

arch/x86: Add compiler flags for auto host CPU

Previously the `CONFIG_MARCH_X86_64_NATIVE` variable did not add
`-march=native` to CFLAGS, the compiler then defaulting to generic
x86_64. This implements the expected functionality.

Signed-off-by: Andrei Tatar <andrei.ttr@gmail.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #861