[PATCH] Fix GCC tests fail for installed toolchain due to ASan, UBSan and TSan testsuites drop GCC_EXEC_PREFIX.

[Maxim Ostapenko]

Hi,

After enabling ASan, TSan and UBSan testsuites for installed toolchain,
many tests started to fail. This is caused by wrong logic in {asan,
ubsan, tsan}_finish
functions. Here, restore_ld_library_path is called, that is wrong,
because it drops some env variables ( GCC_EXEC_PREFIX, LD_LIBRARY_PATH,
etc) to state that was before gcc-dg.exp initialized testing
environment, so installed GCC will be confused to find some needed stuff
later.

Removing restore_ld_library_path from {asan, ubsan, tsan}_finish seems
to fix the issue.

Tested on x86_64-pc-linux-gnu, ok to commit?

-Maxim