Revert "um: clang: Strip out -mno-global-merge from USER_CFLAGS"

This reverts commit 6580c5c1.

This patch is buggy, as noted in the patch linked below. The root cause
has been solved by removing '-mno-global-merge' for the entire kernel.

Link: https://lore.kernel.org/r/20220322173547.677760-1-nathan@kernel.org/


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>