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

(cherry picked from commit f4d93b6761)

sys/conf/files.arm64

@@ -76,7 +76,8 @@ arm64/arm64/uio_machdep.c			standard
 arm64/arm64/uma_machdep.c			standard
 arm64/arm64/undefined.c				standard
 arm64/arm64/unwind.c				optional ddb | kdtrace_hooks | stack
-arm64/arm64/vfp.c				standard
+arm64/arm64/vfp.c				standard \
+	compile-with "${NORMAL_C:N-mgeneral-regs-only}"
 arm64/arm64/vm_machdep.c			standard
 
 arm64/coresight/coresight.c			standard