Use CLOCK_BOOTTIME for Instant

Discussion is ongoing upstream at
rust-lang/rust#87906
about how exactly to make a change like this, but most of the concerns
(backwards compatibility, legacy kernels prior to 2.6.39, non-Linux
OSes) are not applicable to us.

If upstream decideds to keep `Instant` as `CLOCK_MONOTONIC`, we should
figure out an alternate solution for our use of `quiche` before backing
this out.

Bug: 200694560
Change-Id: I188ad48bbaccd2fb0196e8273b307a7bbfc737f1

Author: maurer

patches/rustc-0018-Switch-Instant-to-use-CLOCK_BOOTTIME.patch

@@ -0,0 +1,30 @@
+From 267585dccb068a609c424ffd564abbb0d8a948b9 Mon Sep 17 00:00:00 2001
+From: Matthew Maurer <[email protected]>
+Date: Mon, 9 Aug 2021 19:01:45 -0700
+Subject: [PATCH] Switch Instant to use CLOCK_BOOTTIME
+
+For Linux-like platforms, use CLOCK_BOOTTIME which continues ticking
+during suspend. Without this change, `Duration` between two `Instant`s
+can bear little relation to reality if a suspend took place in between.
+
+Fixes rust-lang#87906
+---
+ library/std/src/sys/unix/time.rs | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/library/std/src/sys/unix/time.rs b/library/std/src/sys/unix/time.rs
+index 7dc09add27f..b8e00a8f2de 100644
+--- a/library/std/src/sys/unix/time.rs
++++ b/library/std/src/sys/unix/time.rs
+@@ -293,7 +293,7 @@ pub struct SystemTime {
+ 
+     impl Instant {
+         pub fn now() -> Instant {
+-            Instant { t: now(libc::CLOCK_MONOTONIC) }
++            Instant { t: now(libc::CLOCK_BOOTTIME) }
+         }
+ 
+         pub const fn zero() -> Instant {
+-- 
+2.33.0.464.g1972c5931b-goog
+