[XCOFF] Enable available_externally linkage for functions.

[XCOFF] Enable available_externally linkage for functions.

Summary: D80642 added support for emitting AvailableExternally Linkage on AIX. However, an assertion of "Trying to get csect representation of this symbol but none was set." occurred when a function is declared as available_externally. This is due to we missing to generate a csect for the function. This patch fixes it.

Reviewed By: hubert.reinterpretcast, shchenz

Differential Revision: https://reviews.llvm.org/D156213

Signed-off-by: Esme Yi <esme.yi@ibm.com>
(cherry picked from commit e83b8a5e711a663c44e80965da5c747e08dea497)