Fix arm64 build after llvm 18.1.3 upgrade

Apparently clang 18 has become more strict about using floating point
registers in inline assembly when -mgeneral-regs-only is used. This
causes sys/arm64/arm64/vfp.c to fail to compile, with "error:
instruction requires: fp-armv8", and "error: expected readable system
register".

To fix it, similar to other files compiled for arm64, disable
-mgeneral-regs-only for this particular file.

PR: 276104
MFC after: 1 month