arm: Handle fixed PIC register in require_pic_register (PR target/115485)

arm: Handle fixed PIC register in require_pic_register (PR target/115485)

Commit r9-4307-g89d7557202d25a forgot to accept a fixed PIC register
when extending the assert in require_pic_register.

arm_pic_register can be set explicitly by the user
(e.g. -mpic-register=r9) or implicitly as the default value with
-fpic/-fPIC/-fPIE and -mno-pic-data-is-text-relative -mlong-calls, and
we want to use/accept it when recording cfun->machine->pic_reg as used
to be the case.

	PR target/115485
	gcc/
	* config/arm/arm.cc (require_pic_register): Fix typos in
	comment. Handle fixed arm_pic_register.

	gcc/testsuite/
	* g++.target/arm/pr115485.C: New test.