Change encodeU/SLEB128 to pad to certain number of bytes

Previously the 'Padding' argument was the number of padding
bytes to add. However most callers that use 'Padding' know
how many overall bytes they need to write.  With the previous
code this would mean encoding the LEB once to find out how
many bytes it would occupy and then using this to calulate
the 'Padding' value.

See: https://reviews.llvm.org/D36595

Differential Revision: https://reviews.llvm.org/D37494

llvm-svn: 313393