[bindings/go] Fix building on 32-bit systems (ARM etc.)

Summary:
The patch in https://reviews.llvm.org/D53883 (by me) fails to build on 32-bit systems like ARM. Fix the array size to be less ridiculously large. 2<<20 should still be enough for all practical purposes.

Bug: https://bugs.llvm.org/show_bug.cgi?id=40426

Reviewers: whitequark, pcc

Reviewed By: whitequark

Subscribers: javed.absar, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58030

llvm-svn: 354207