[MC] Error for .globl/.local which change the symbol binding and warn for .weak

GNU as let .weak override .globl since binutils-gdb
5ca547dc2399a0a5d9f20626d4bf5547c3ccfddd (1996) while MC lets the last
directive win (PR38921).

This caused an issue to Linux's powerpc port which has been fixed by
http://git.kernel.org/linus/968339fad422a58312f67718691b717dac45c399

Binding overriding is error-prone. This patch disallows a changed binding.
(https://sourceware.org/pipermail/binutils/2020-March/000299.html )

Our behavior regarding `.globl x; .weak x` matches GNU as. Such usage is
still suspicious but we issue a warning for now. We may upgrade it to an
error in the future.

Reviewed By: jhenderson, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D90108