[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2021-04-12  Will Schmidt  <will_schmidt@vnet.ibm.com>
+
+	* gdb.arch/powerpc-vsx-regs.exp: Initialize vs* doublewords.
+
 2021-04-12  Will Schmidt  <will_schmidt@vnet.ibm.com>
 
 	* gdb.arch/powerpc-plxv-nonrel.s:  Testcase using

gdb/testsuite/gdb.arch/vsx-regs.exp

@@ -89,7 +89,18 @@ if {$endianness == "big"} {
 
 set float_register ".raw 0xdeadbeefdeadbeef."
 
-# First run the F0~F31/VS0~VS31 tests
+# Note that the F0-F31 registers are shared with the doubleword 0 portion of
+# the VS0-VS31 registers, the doubleword 1 portions of VS* remain unchanged
+# after updates to F*.
+# Since dl_main uses some VS* registers, and per inspection their values are
+# no longer zero when our test reaches main(), we need to explicitly
+# initialize the doubleword1 portions before we run our tests against
+# values currently in those registers.
+
+# 0: Initialize the (doubleword 1) portion of the VS0-VS31 registers.
+for {set i 0} {$i < 32} {incr i 1} {
+    gdb_test_no_output "set \$vs$i.v2_double\][0\] = 0"
+}
 
 # 1: Set F0~F31 registers and check if it reflects on VS0~VS31.
 for {set i 0} {$i < 32} {incr i 1} {