cad/openvsp: Unbreak port

Port was broken after last compilers updates in 11 and 12.
Clang is now able to compile the port but we need to patch as_callfunc_x86.cpp
unconditionally, otherwise clang complains (or dies, depending on the version)
with:

this directive must appear between .cfi_startproc and .cfi_endproc directives

Also use localbase to simplify flags handling.

Reported by: pkg-fallout