[NFC][AIX][XCOFF] if the size of Csect is zero, the Csect do not need write any data into sections

SUMMARY:

if the size of Csect is zero, the Csect do not need write any data into sections
for example, the TOC Csect has zero size, it do not need invoke a
Asm.writeSectionData(W.OS, Csect.MCCsect, Layout);

Reviewers: daltenty
Subscribers: rupprecht, seiyai,hiraditya

Differential Revision: https://reviews.llvm.org/D71120