PowerPC, fix test gdb.arch/altivec-regs.exp

PowerPC, fix test gdb.arch/altivec-regs.exp

The test fails on Power 10 with the RHEL9 distro.  It also fails on
Power 9.

The test set a the breakpoint in main that stops at line:
a = 9; /* start here */.  The test then sets a break point at the same
line where it wants to start the test and does a continue.  GDB does not
stop again on the same line where it is stopped, but rather continues to
the end of the program.

Initialize variable A to zero so the break on main will stop before setting
a break point on line a = 9; /* start here */.

Make the match on the breakpoint number generic.

Patch has been tested on Power 10 with RHEL 9, Power 10 with Ubuntu 22.04,
and Power 9 with Fedora 36 with no regression failures.