runtime: add symbol for AT_FDCWD on Linux amd64 and mips64x

Also order the syscall number list by numerically for mips64x.

Follow-up for CL 92895.

Change-Id: I5f01f8c626132a06160997fce8a2aef0c486bb1c
Reviewed-on: https://go-review.googlesource.com/93616
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_linux_amd64.s

@@ -10,6 +10,8 @@
 #include "go_tls.h"
 #include "textflag.h"
 
+#define AT_FDCWD -100
+
 #define SYS_read		0
 #define SYS_write		1
 #define SYS_close		3
@@ -66,7 +68,7 @@ TEXT runtime·exitThread(SB),NOSPLIT,$0-8
 
 TEXT runtime·open(SB),NOSPLIT,$0-20
 	// This uses openat instead of open, because Android O blocks open.
-	MOVL	$-100, DI // AT_FDCWD, so this acts like open
+	MOVL	$AT_FDCWD, DI // AT_FDCWD, so this acts like open
 	MOVQ	name+0(FP), SI
 	MOVL	mode+8(FP), DX
 	MOVL	perm+12(FP), R10

src/runtime/sys_linux_mips64x.s

@@ -13,10 +13,11 @@
 #include "go_tls.h"
 #include "textflag.h"
 
+#define AT_FDCWD -100
+
 #define SYS_exit		5058
 #define SYS_read		5000
 #define SYS_write		5001
-#define SYS_openat		5247
 #define SYS_close		5003
 #define SYS_getpid		5038
 #define SYS_kill		5060
@@ -42,6 +43,7 @@
 #define SYS_exit_group		5205
 #define SYS_epoll_create	5207
 #define SYS_epoll_ctl		5208
+#define SYS_openat		5247
 #define SYS_epoll_pwait		5272
 #define SYS_clock_gettime	5222
 #define SYS_epoll_create1	5285
@@ -68,7 +70,7 @@ TEXT runtime·exitThread(SB),NOSPLIT|NOFRAME,$0-8
 
 TEXT runtime·open(SB),NOSPLIT|NOFRAME,$0-20
 	// This uses openat instead of open, because Android O blocks open.
-	MOVW	$-100, R4 // AT_FDCWD, so this acts like open
+	MOVW	$AT_FDCWD, R4 // AT_FDCWD, so this acts like open
 	MOVV	name+0(FP), R5
 	MOVW	mode+8(FP), R6
 	MOVW	perm+12(FP), R7