Add target hook for asm operand validation.

Add target hook for asm operand validation.

The PowerPC has noraml loads and stores and then it has prefixed loads and
stores.  The prefixed loads and stores have different names and format
than the normal loads and stores.  This patch adds a new hook that is run
in the passes before reload to ensure that no prefixed memory is generated
by optimizations.

This patch is all of the machine independent changes to to add a hook to
prevent asm from generating prefixed loads/stores with the '%m' constraint
on the PowerPC.  The next patch will contain the PowerPC specific patches.

2021-07-31  Michael Meissner  <meissner@linux.ibm.com>
gcc/
	PR target/98519
	* doc/tm.texi (TARGET_MD_ASM_OPERAND_P): Document.
	* doc/tm.texi.in (TARGET_MD_ASM_OPERAND_P): Document.
	* target.def (md_asm_operand_p): New target hook.
	* targhooks.c (default_md_asm_operand_p): New default target
	hook.
	* targhooks.h (default_md_asm_operand_p): New declaration.