Pick an alternate static chain regnum and document that
the port needs to be configured to issue -ffixed-r18 by
default.  The current definition of TARGET_OS_USES_R18 is
not honored by the common part of the back-end.

2020-11-04  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/aarch64/aarch64-vxworks.h (TARGET_OS_USES_R18):
	Remove definition.
	(STATIC_CHAIN_REGNUM): Redefine to 9.