rs6000: Adding missed ISA 3.0 atomic memory operation instructions

Changes to amo.h include the addition of the following load atomic
operations: Compare and Swap Not Equal, Fetch and Increment Bounded,
Fetch and Increment Equal, and Fetch and Decrement Bounded.
Additionally, Store Twin is added for store atomic operations.

2026-05-19  Peter Bergner  <bergner@linux.ibm.com>
	    Jeevitha Palanisamy  <jeevitha@linux.ibm.com>

gcc/
	* config/rs6000/amo.h: Add missing atomic memory operations.
	* doc/extend.texi (PowerPC Atomic Memory Operation Functions): Document
	new functions.

gcc/testsuite/
	* gcc.target/powerpc/amo3.c: New test.
	* gcc.target/powerpc/amo4.c: Likewise.
	* gcc.target/powerpc/amo5.c: Likewise.
	* gcc.target/powerpc/amo6.c: Likewise.
	* gcc.target/powerpc/amo7.c: Likewise.

(cherry picked from commit 6953b4ea86a669ef6ad87900bbbd6db9b970f9d2)