gdb.trace: Surround $call_insn with \y in entry-values.exp

The PPC64 tracepoint patch added \y at the end of the call_insn pattern -
without that, it embarassed itself and matched the 'bl' in "Dump of
assem*bl*er code for function" as the powerpc call opcode.  Since that
sounds like a generally good idea, I've added \y before and after
call_insn for every target.  As a result, I had to change x86_64's mnemonic
to 'callq'.

gdb/testsuite/ChangeLog:

	* gdb.trace/entry-values.exp: Surround $call_insn with '\y',
	change x86_64 call_insn to 'callq'.