gdb: fix powerpc disassembly tests

This patch fixes test failures power8 and power9 caused by changes on
opcodes:

The dissasembler does not emit whitespace for instructions
anymore (c2b1c275)

The dissasembler generates extended mnemonics for some instructions
instead (aae9718e)

The ldmx instruction was removed. This instruction was never
implemented (6fbc939c)

gdb/testsuite/ChangeLog:
2020-02-03  Rogerio A. Cardoso  <rcardoso@linux.ibm.com>

	* gdb.arch/powerpc-power8.exp: Delete trailing whitespace of
	tbegin., tend. instructions. Replace bctar-, bctar+, bctarl-,
	bctarl+ extended mnemonics when avaliable by bgttar, bnstarl,
	blttar, bnetarl.
	* gdb.arch/powerpc-power8.s: Fix comments. Fix instructions
	binary for blttar, bnetarl.
	* gdb.arch/powerpc-power9.exp: Delete trailing whitespace of
	wait instruction. Delete ldmx test.
	* gdb.arch/powerpc-power9.s: Delete ldmx instruction.