[MC][test] Fix non-portable GNU diff option

Summary: This patch replaces the non-portable GNU diff option `--strip-trailing-cr` with the POSIX `-b` option in two test files.

Reviewers: daltenty, jasonliu

Reviewed By: daltenty

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72745

llvm/test/MC/ARM/preserve-comments-arm.s

@@ -1,6 +1,6 @@
 	@RUN: llvm-mc -preserve-comments -n -triple arm-eabi < %s > %t
 	@RUN: sed 's/#[C]omment/@Comment/g' %s > %t2
-	@RUN: diff --strip-trailing-cr %t %t2
+	@RUN: diff -b %t %t2
 	.text
 
 	mov	r0, r0

llvm/test/MC/AsmParser/preserve-comments.s

@@ -1,5 +1,5 @@
 	#RUN: llvm-mc -preserve-comments -n -triple i386-linux-gnu < %s > %t
-	#RUN: diff --strip-trailing-cr %s %t
+	#RUN: diff -b %s %t
 	.text
 
 foo:	#Comment here