port: add 'notext' keyword linker support

port: add 'notext' keyword linker support

Check if the linker support the 'notext' keyword to allow relocations
against read-only segments. GNU ld defaults to notext but LLVM's ld does
not, both linkers support the keyword.

This is required for the plugins section symbols.

Change-Id: I2b5f5c77865267e35c529dd17a0298cd4405303d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>