[mlir] LLVM dialect: support globals without linkage keyword, assuming 'external'

Similarly to actual LLVM IR, and to `llvm.mlir.func`, allow the custom syntax
of `llvm.mlir.global` to omit the linkage keyword. If omitted, the linkage is
assumed to be external. This makes the modeling of globals in the LLVM dialect
more consistent, both within the dialect and with LLVM IR.

Differential Revision: https://reviews.llvm.org/D78096