syscall: fix system calls with misaligned arguments on freebsd/arm

This CL enables the current tree to work with FreeBSD 10-STABLE
on ARM EABI platforms, though there are still a few test fails.

Also updates documentation.

LGTM=iant
R=iant, dave
CC=golang-codereviews
https://golang.org/cl/61060044

src/pkg/syscall/asm_freebsd_arm.s

@@ -8,13 +8,13 @@
 // System call support for ARM, FreeBSD
 //
 
-// func Syscall(trap int32, a1, a2, a3 int32) (r1, r2, err int32);
-// func Syscall6(trap int32, a1, a2, a3, a4, a5, a6 int32) (r1, r2, err int32);
-// func Syscall9(trap int32, a1, a2, a3, a4, a5, a6, a7, a8, a9 int64) (r1, r2, err int32)
+// func Syscall(trap, a1, a2, a3 uintptr) (r1, r2, errno uintptr);
+// func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, errno uintptr);
+// func Syscall9(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2, errno uintptr)
 
 TEXT	·Syscall(SB),NOSPLIT,$0-28
 	BL runtime·entersyscall(SB)
-	MOVW 0(FP), R7 // sigcall num
+	MOVW 0(FP), R7 // syscall number
 	MOVW 4(FP), R0 // a1
 	MOVW 8(FP), R1 // a2
 	MOVW 12(FP), R2 // a3
@@ -23,69 +23,71 @@ TEXT	·Syscall(SB),NOSPLIT,$0-28
 	BCS error
 	MOVW R0, 16(FP) // r1
 	MOVW R1, 20(FP) // r2
-	MOVW R2, 24(FP) // err
+	MOVW R2, 24(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 error:
 	MOVW $-1, R3
 	MOVW R3, 16(FP) // r1
 	MOVW R2, 20(FP) // r2
-	MOVW R0, 24(FP) // err
+	MOVW R0, 24(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-40
 	BL runtime·entersyscall(SB)
-	MOVW 0(FP), R7 // sigcall num
+	MOVW 0(FP), R7 // syscall number
 	MOVW 4(FP), R0 // a1
 	MOVW 8(FP), R1 // a2
 	MOVW 12(FP), R2 // a3
 	MOVW 16(FP), R3 // a4
-	ADD $24, R13 // a5 to a6 are passed on stack
+	MOVW R13, R4
+	MOVW $20(FP), R13 // a5 to a6 are passed on stack
 	SWI $0 // syscall
-	SUB $24, R13
+	MOVW R4, R13
 	MOVW $0, R2
 	BCS error6
 	MOVW R0, 28(FP) // r1
 	MOVW R1, 32(FP) // r2
-	MOVW R2, 36(FP) // err
+	MOVW R2, 36(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 error6:
 	MOVW $-1, R3
 	MOVW R3, 28(FP) // r1
 	MOVW R2, 32(FP) // r2
-	MOVW R0, 36(FP) // err
+	MOVW R0, 36(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 
 TEXT	·Syscall9(SB),NOSPLIT,$0-52
 	BL runtime·entersyscall(SB)
-	MOVW 0(FP), R7 // sigcall num
+	MOVW 0(FP), R7 // syscall number
 	MOVW 4(FP), R0 // a1
 	MOVW 8(FP), R1 // a2
 	MOVW 12(FP), R2 // a3
 	MOVW 16(FP), R3 // a4
-	ADD $24, R13 // a5 to a9 are passed on stack
+	MOVW R13, R4
+	MOVW $20(FP), R13 // a5 to a9 are passed on stack
 	SWI $0 // syscall
-	SUB $24, R13
+	MOVW R4, R13
 	MOVW $0, R2
 	BCS error9
 	MOVW R0, 40(FP) // r1
 	MOVW R1, 44(FP) // r2
-	MOVW R2, 48(FP) // err
+	MOVW R2, 48(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 error9:
 	MOVW $-1, R3
 	MOVW R3, 40(FP) // r1
 	MOVW R2, 44(FP) // r2
-	MOVW R0, 48(FP) // err
+	MOVW R0, 48(FP) // errno
 	BL runtime·exitsyscall(SB)
 	RET
 
 TEXT	·RawSyscall(SB),NOSPLIT,$0-28
-	MOVW 0(FP), R7 // sigcall num
+	MOVW 0(FP), R7 // syscall number
 	MOVW 4(FP), R0 // a1
 	MOVW 8(FP), R1 // a2
 	MOVW 12(FP), R2 // a3
@@ -94,33 +96,34 @@ TEXT	·RawSyscall(SB),NOSPLIT,$0-28
 	BCS errorr
 	MOVW R0, 16(FP) // r1
 	MOVW R1, 20(FP) // r2
-	MOVW R2, 24(FP) // err
+	MOVW R2, 24(FP) // errno
 	RET
 errorr:
 	MOVW $-1, R3
 	MOVW R3, 16(FP) // r1
 	MOVW R2, 20(FP) // r2
-	MOVW R0, 24(FP) // err
+	MOVW R0, 24(FP) // errno
 	RET
 
 TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
-	MOVW 0(FP), R7 // sigcall num
+	MOVW 0(FP), R7 // syscall number
 	MOVW 4(FP), R0 // a1
 	MOVW 8(FP), R1 // a2
 	MOVW 12(FP), R2 // a3
 	MOVW 16(FP), R3 // a4
-	ADD $24, R13 // a5 to a6 are passed on stack
+	MOVW R13, R4
+	MOVW $20(FP), R13 // a5 to a6 are passed on stack
 	SWI $0 // syscall
-	SUB $24, R13
+	MOVW R4, R13
 	MOVW $0, R2
 	BCS errorr6
 	MOVW R0, 28(FP) // r1
 	MOVW R1, 32(FP) // r2
-	MOVW R2, 36(FP) // err
+	MOVW R2, 36(FP) // errno
 	RET
 errorr6:
 	MOVW $-1, R3
 	MOVW R3, 28(FP) // r1
 	MOVW R2, 32(FP) // r2
-	MOVW R0, 36(FP) // err
+	MOVW R0, 36(FP) // errno
 	RET