Use PIE when compiling tests on Linux.

This is an attempt to make linux_msan bot less flaky which is suggested by the error message:
FATAL: Make sure to compile with -fPIE and to link with -pie.

Bug: None
Change-Id: I43f4e5c10d9a4c42a8ef5a67bf77806f0c09bf98
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/6111093
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>

BUILD.gn

@@ -395,6 +395,8 @@ if (libyuv_include_tests) {
 
     if (is_linux || is_chromeos) {
       cflags = [ "-fexceptions" ]
+      # For enabling ASLR.
+      ldflags = [ "-pie" ]
     }
     if (is_ios) {
       configs -= [ "//build/config/compiler:default_symbols" ]