feat(amd64): encode the GOROOT instruction families

Assisted-by: GLM 5.3 Flash

testdata/verify/system_amd64.s

@@ -0,0 +1,76 @@
+// System, string-primitive and x87 families: flag register moves, the
+// serialising instructions, MOVS/STOS, the MXCSR pair, scalar float-to-int
+// conversions and FMOVD.  Every result is folded back so no instruction is
+// dead.
+
+#include "textflag.h"
+
+// func system(x uint64) uint64
+TEXT ·system(SB), NOSPLIT, $0-16
+	MOVQ    x+0(FP), AX
+	PUSHFQ
+	POPFQ
+	CPUID
+	RDTSC
+	RDTSCP
+	SYSCALL
+	XGETBV
+	PAUSE
+	LFENCE
+	MFENCE
+	SFENCE
+	UNDEF
+	XORQ    AX, BX
+	MOVQ    BX, ret+8(FP)
+	RET
+
+// func stringprim(p *byte, n int) uint64
+TEXT ·stringprim(SB), NOSPLIT, $0-24
+	MOVQ      p+0(FP), DI
+	MOVQ      n+8(FP), CX
+	LEAQ      buf<>(SB), AX
+	MOVQ      AX, SI
+	CLD
+	MOVSB
+	MOVSW
+	MOVSL
+	MOVSQ
+	STOSB
+	STOSQ
+	STOSL
+	STOSW
+	MOVQ      DI, ret+16(FP)
+	RET
+
+DATA buf<>+0x00(SB)/8, $0
+
+GLOBL buf<>(SB), NOPTR, $8
+
+// func intgate(x uint64) uint64
+TEXT ·intgate(SB), NOSPLIT, $0-16
+	MOVQ    x+0(FP), AX
+	INT     $3
+	MOVQ    AX, ret+8(FP)
+	RET
+
+// func fpmxcsr(x float64, csr *uint32) int64
+TEXT ·fpmxcsr(SB), NOSPLIT, $0-24
+	MOVQ      x+0(FP), X0
+	MOVQ      csr+8(FP), AX
+	STMXCSR   (AX)
+	LDMXCSR   (AX)
+	CVTSD2SL  X0, CX
+	CVTTSD2SQ X0, DX
+	MOVL      (AX), SI
+	MOVQ      SI, ret+8(FP)
+	RET
+
+// func fmove(p *float64) float64
+TEXT ·fmove(SB), NOSPLIT, $0-16
+	MOVQ  p+0(FP), AX
+	FMOVD (AX), F0
+	FMOVD F0, F1
+	FMOVD F0, (AX)
+	MOVQ  (AX), AX
+	MOVQ  AX, ret+8(FP)
+	RET