runtime: use pselect6 for usleep on linux/amd64 and linux/arm

Android O black-lists the select system call because its libc, Bionic,
does not use this system call. Replace our use of select with pselect6
(which is allowed) on the platforms that support targeting Android.
linux/arm64 already uses pselect6 because there is no select on arm64,
so only linux/amd64 and linux/arm need changing. pselect6 has been
available since Linux 2.6.16, which is before Go's minimum
requirement.

Fixes #20409.

Change-Id: Ic526b5b259a9e01d2f145a1f4d2e76e8c49ce809
Reviewed-on: https://go-review.googlesource.com/43641
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_linux_amd64.s

@@ -82,15 +82,18 @@ TEXT runtime·usleep(SB),NOSPLIT,$16
 	MOVL	$1000000, CX
 	DIVL	CX
 	MOVQ	AX, 0(SP)
-	MOVQ	DX, 8(SP)
+	MOVL	$1000, AX	// usec to nsec
+	MULL	DX
+	MOVQ	AX, 8(SP)
 
-	// select(0, 0, 0, 0, &tv)
+	// pselect6(0, 0, 0, 0, &ts, 0)
 	MOVL	$0, DI
 	MOVL	$0, SI
 	MOVL	$0, DX
 	MOVL	$0, R10
 	MOVQ	SP, R8
-	MOVL	$23, AX
+	MOVL	$0, R9
+	MOVL	$270, AX
 	SYSCALL
 	RET
 

src/runtime/sys_linux_arm.s

@@ -36,7 +36,7 @@
 #define SYS_gettid (SYS_BASE + 224)
 #define SYS_tkill (SYS_BASE + 238)
 #define SYS_sched_yield (SYS_BASE + 158)
-#define SYS_select (SYS_BASE + 142) // newselect
+#define SYS_pselect6 (SYS_BASE + 335)
 #define SYS_ugetrlimit (SYS_BASE + 191)
 #define SYS_sched_getaffinity (SYS_BASE + 242)
 #define SYS_clock_gettime (SYS_BASE + 263)
@@ -388,13 +388,16 @@ TEXT runtime·usleep(SB),NOSPLIT,$12
 	MOVW	usec+0(FP), R0
 	CALL	runtime·usplitR0(SB)
 	MOVW	R0, 4(R13)
+	MOVW	$1000, R0	// usec to nsec
+	MUL	R0, R1
 	MOVW	R1, 8(R13)
 	MOVW	$0, R0
 	MOVW	$0, R1
 	MOVW	$0, R2
 	MOVW	$0, R3
 	MOVW	$4(R13), R4
-	MOVW	$SYS_select, R7
+	MOVW	$0, R5
+	MOVW	$SYS_pselect6, R7
 	SWI	$0
 	RET