external/boringssl: Sync to b2ff2623a88a65fd4db42d3820f3d8c64e8ab180.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/6d50f475e319de153a43e1dba5a1beca95948c63..b2ff2623a88a65fd4db42d3820f3d8c64e8ab180

Change-Id: I649281e093369d99e863b4882a2ff6a5ad8a64d1
Test: ATP's cts/libcore/gce-net (go/gce-net)
diff --git a/src/crypto/bn/asm/x86_64-mont5.pl b/src/crypto/bn/asm/x86_64-mont5.pl
index 61dd902..61fde2d 100755
--- a/src/crypto/bn/asm/x86_64-mont5.pl
+++ b/src/crypto/bn/asm/x86_64-mont5.pl
@@ -1852,6 +1852,7 @@
 
 .align	32
 .L8x_tail_done:
+	xor	%rax,%rax
 	add	(%rdx),%r8		# can this overflow?
 	adc	\$0,%r9
 	adc	\$0,%r10
@@ -1859,10 +1860,8 @@
 	adc	\$0,%r12
 	adc	\$0,%r13
 	adc	\$0,%r14
-	adc	\$0,%r15		# can't overflow, because we
-					# started with "overhung" part
-					# of multiplication
-	xor	%rax,%rax
+	adc	\$0,%r15
+	adc	\$0,%rax
 
 	neg	$carry
 .L8x_no_tail:
@@ -3248,6 +3247,7 @@
 
 .align	32
 .Lsqrx8x_tail_done:
+	xor	%rax,%rax
 	add	24+8(%rsp),%r8		# can this overflow?
 	adc	\$0,%r9
 	adc	\$0,%r10
@@ -3255,10 +3255,8 @@
 	adc	\$0,%r12
 	adc	\$0,%r13
 	adc	\$0,%r14
-	adc	\$0,%r15		# can't overflow, because we
-					# started with "overhung" part
-					# of multiplication
-	mov	$carry,%rax		# xor	%rax,%rax
+	adc	\$0,%r15
+	adc	\$0,%rax
 
 	sub	16+8(%rsp),$carry	# mov 16(%rsp),%cf
 .Lsqrx8x_no_tail:			# %cf is 0 if jumped here
@@ -3273,7 +3271,7 @@
 	adc	8*5($tptr),%r13
 	adc	8*6($tptr),%r14
 	adc	8*7($tptr),%r15
-	adc	%rax,%rax		# top-most carry
+	adc	\$0,%rax		# top-most carry
 
 	mov	32+8(%rsp),%rbx		# n0
 	mov	8*8($tptr,%rcx),%rdx	# modulo-scheduled "%r8"