[lsan] Reenable lsan tests on ARM bots

This patch addresses pr32636. Enable lsan tests on ARM bots filtering out Thumb targets.
Tested locally on ARM Arndale board in two configurations:

1) CFLAGS="-march=armv7-a"
	Testing Time: 37.57s
	Expected Passes    : 69
	Unsupported Tests  : 7

2) CFLAGS="-march=armv7-a -mthumb"
	Testing Time: 0.16s
	Unsupported Tests  : 76

Differential Revision: https://reviews.llvm.org/D32007

llvm-svn: 300194