[RuntimeDyld] Make sure code-sections aren't under-aligned.

Code-section alignment should be at least as high as the minimum
stub alignment. If the section alignment is lower it can cause
padding to be emitted resulting in alignment errors if the section
is mapped to a higher alignment on the target.

E.g. If a text section with a 4-byte alignment gets 4-bytes of
padding to guarantee 8-byte alignment for stubs but is re-mapped to
an 8-byte alignment on the target, the 4-bytes of padding will push
the stubs to 4-byte alignment causing a crash.

No test case: There is currently no way to control host section
alignment in llvm-rtdyld. This could be made testable by adding
a custom memory manager. I'll look at that in a follow-up patch.

llvm-svn: 245031