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

sys/arm64/arm64/vfp.c

@@ -110,6 +110,7 @@ vfp_store(struct vfpstate *state)
 
 	vfp_state = state->vfp_regs;
 	__asm __volatile(
+	    ".arch_extension fp\n"
 	    "mrs	%0, fpcr		\n"
 	    "mrs	%1, fpsr		\n"
 	    "stp	q0,  q1,  [%2, #16 *  0]\n"
@@ -128,6 +129,7 @@ vfp_store(struct vfpstate *state)
 	    "stp	q26, q27, [%2, #16 * 26]\n"
 	    "stp	q28, q29, [%2, #16 * 28]\n"
 	    "stp	q30, q31, [%2, #16 * 30]\n"
+	    ".arch_extension nofp\n"
 	    : "=&r"(fpcr), "=&r"(fpsr) : "r"(vfp_state));
 
 	state->vfp_fpcr = fpcr;
@@ -145,6 +147,7 @@ vfp_restore(struct vfpstate *state)
 	fpsr = state->vfp_fpsr;
 
 	__asm __volatile(
+	    ".arch_extension fp\n"
 	    "ldp	q0,  q1,  [%2, #16 *  0]\n"
 	    "ldp	q2,  q3,  [%2, #16 *  2]\n"
 	    "ldp	q4,  q5,  [%2, #16 *  4]\n"
@@ -163,6 +166,7 @@ vfp_restore(struct vfpstate *state)
 	    "ldp	q30, q31, [%2, #16 * 30]\n"
 	    "msr	fpcr, %0		\n"
 	    "msr	fpsr, %1		\n"
+	    ".arch_extension nofp\n"
 	    : : "r"(fpcr), "r"(fpsr), "r"(vfp_state));
 }
 

sys/conf/files.arm64

@@ -82,8 +82,7 @@ arm64/arm64/uma_machdep.c			standard
 arm64/arm64/undefined.c				standard
 arm64/arm64/unwind.c				optional ddb | kdtrace_hooks | stack \
 	compile-with "${NORMAL_C:N-fsanitize*:N-fno-sanitize*}"
-arm64/arm64/vfp.c				standard \
-	compile-with "${NORMAL_C:N-mgeneral-regs-only}"
+arm64/arm64/vfp.c				standard
 arm64/arm64/vm_machdep.c			standard
 
 arm64/coresight/coresight.c			standard