Use empty symbol name for XCOFF text csect

When generating XCOFF, the compiler generates a csect with an internal
name.  Each function results in a label within the csect.  This patch
replaces the internal name ".text" with an empty string "".  This avoids
adding special code to handle a function text() in the source file, and
works better with some XCOFF tools that are confused when the csect and
the first function have the same address.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D154854