[mach-o]: tighten up diagnostics for -image_base option

The provided base must also be a multiple of the system's page size, which is a
reasonable enough demand.

Also check the other diagnostics more thoroughly.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217577 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o]: support optional "0x" prefix for -image_base

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217578 91177308-0d34-0410-b5e6-96231b3b80d8