gdb fix for catch-syscall.exp

gdb fix for catch-syscall.exp

Remove check_continue "execve" from Proc test_catch_syscall_execve.

The check_continue proceedure checs that the command, execve, starts and
checks for the return from the command.  The execve command starts a new
program and thus the return from the command causing the test to fail.

The call to proc check_continue "execve" is removed and replaced with
just the call to check_call_to_syscall "execve" to verify the command
executed.  The next test in proc test_catch_syscall_execve verifies that
the new program started and hit the break point in main.

Update the check for the PowerPC architecture.  Power Little Endian systems
include "le" in the name.  The istarget "power64-*-linux*" check fails to
match LE sytems.  The expected string is updated to capture both Big Endian
and Little Endian systems.  Power 10 LE istarget prints as:
powerpc64le-unknown-linux-gnu.

This patch fixes three failures and the error:

    ERROR: can't read "arch1": no such variable

Patch tested on Power 10 ppc64le GNU/Linux platform.