LoongArch: Prepare for assemblers with proper FCSR class support

The GNU assembler (as of 2.40) mis-treats FCSR operands as GPRs, but
the LLVM IAS does not. Probe for this and refer to FCSRs as "$fcsrNN"
if support is present.
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -241,6 +241,9 @@ config SCHED_OMIT_FRAME_POINTER
 config AS_HAS_EXPLICIT_RELOCS
 	def_bool $(as-instr,x:pcalau12i \$t0$(comma)%pc_hi20(x))
 
+config AS_HAS_FCSR_CLASS
+	def_bool $(as-instr,movfcsr2gr \$t0$(comma)\$fcsr0)
+
 menu "Kernel type and options"
 
 source "kernel/Kconfig.hz"

arch/loongarch/include/asm/fpregdef.h

@@ -40,6 +40,7 @@
 #define fs6	$f30
 #define fs7	$f31
 
+#ifndef CONFIG_AS_HAS_FCSR_CLASS
 /*
  * Current binutils expects *GPRs* at FCSR position for the FCSR
  * operation instructions, so define aliases for those used.
@@ -48,5 +49,11 @@
 #define fcsr1	$r1
 #define fcsr2	$r2
 #define fcsr3	$r3
+#else
+#define fcsr0	$fcsr0
+#define fcsr1	$fcsr1
+#define fcsr2	$fcsr2
+#define fcsr3	$fcsr3
+#endif
 
 #endif /* _ASM_FPREGDEF_H */

arch/loongarch/include/asm/loongarch.h

@@ -1441,11 +1441,18 @@ __BUILD_CSR_OP(tlbidx)
 #define EXCCODE_INT_START	64
 #define EXCCODE_INT_END		(EXCCODE_INT_START + EXCCODE_INT_NUM - 1)
 
-/* FPU register names */
+/* FPU Status Register Names */
+#ifndef CONFIG_AS_HAS_FCSR_CLASS
 #define LOONGARCH_FCSR0	$r0
 #define LOONGARCH_FCSR1	$r1
 #define LOONGARCH_FCSR2	$r2
 #define LOONGARCH_FCSR3	$r3
+#else
+#define LOONGARCH_FCSR0	$fcsr0
+#define LOONGARCH_FCSR1	$fcsr1
+#define LOONGARCH_FCSR2	$fcsr2
+#define LOONGARCH_FCSR3	$fcsr3
+#endif
 
 /* FPU Status Register Values */
 #define FPU_CSR_RSVD	0xe0e0fce0