Fix arm64 build after llvm 18.1.3 upgrade (take 2)

Instead of compiling the whole sys/arm64/arm64/vfp.c file without
-mgeneral-regs-only, which might have unwanted side effects, add
".arch_extension fp" / ".arch_extension nofp" pairs to the inline
assembly.

PR: 276104
Suggested by: andrew
MFC after: 1 month