[AIX][XCOFF][NFC] Change the SectionLen field name of CSect Auxiliary entry to SectionOrLength.

Summary:
According the the XCOFF document,
If
Then
XTY_SD
x_scnlen contains the csect length.
XTY_LD
x_scnlen contains the symbol table index of the containing csect.
XTY_CM
x_scnlen contains the csect length.
XTY_ER
x_scnlen contains 0.

Change the SectionLen member name to SectionOrLength is more reasonable.

Authored By: DiggerLin

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D68650

llvm-svn: 374179