[PATCH,rs6000] Fix vsx-regs.exp testcase failure

Hi,
  This test exercise updates to the F* and VS* registers
and verifies updates to the same.  Note that the registers
overlap; the doubleword[1] portion of any VS0-VS31
register contains the F0-F31 register contents, so any updates
to one can be measured in the other.

Per a brief investigation, we see that dl_main() currently
uses some VSX instructions, so the VS* values are not
going to be zero when this testcase reaches main, where these
tests begin.  The test harness does not explicitly
initialize the full VS* values, so the first test loop
that updates the F* values means our VS* values are
uninitalized and will fail the first set of checks.
This update explicitly initializes the doubleword[0] portion
of the VS* registers, to allow this test to succeed.

2021-04-12  Will Schmidt  <will_schmidt@vnet.ibm.com>

gdb/testsuite/ChangeLog:
        * gdb.arch/vsx-regs.exp: Initialize vs* doublewords.