arm: fix bootstrap after MVE changes

The recent commits for MVE on Saturday have broken armhf bootstrap due to a
-Werror false positive:

    inlined from 'virtual rtx_def* {anonymous}::vstrq_scatter_base_impl::expand(arm_mve::function_expander&) const' at /gcc/config/arm/arm-mve-builtins-base.cc:352:17:
./genrtl.h:38:16: error: 'new_base' may be used uninitialized [-Werror=maybe-uninitialized]
   38 |   XEXP (rt, 1) = arg1;
/gcc/config/arm/arm-mve-builtins-base.cc: In member function 'virtual rtx_def* {anonymous}::vstrq_scatter_base_impl::expand(arm_mve::function_expander&) const':
/gcc/config/arm/arm-mve-builtins-base.cc:311:26: note: 'new_base' was declared here
  311 |     rtx insns, base_ptr, new_base;
      |                          ^~~~~~~~
In function 'rtx_def* init_rtx_fmt_ee(rtx, machine_mode, rtx, rtx)',
    inlined from 'rtx_def* gen_rtx_fmt_ee_stat(rtx_code, machine_mode, rtx, rtx)' at ./genrtl.h:50:26,
    inlined from 'virtual rtx_def* {anonymous}::vldrq_gather_base_impl::expand(arm_mve::function_expander&) const' at /gcc/config/arm/arm-mve-builtins-base.cc:527:17:
./genrtl.h:38:16: error: 'new_base' may be used uninitialized [-Werror=maybe-uninitialized]
   38 |   XEXP (rt, 1) = arg1;
/gcc/config/arm/arm-mve-builtins-base.cc: In member function 'virtual rtx_def* {anonymous}::vldrq_gather_base_impl::expand(arm_mve::function_expander&) const':
/gcc/config/arm/arm-mve-builtins-base.cc:486:26: note: 'new_base' was declared here
  486 |     rtx insns, base_ptr, new_base;

To fix it I just initialize the value.

gcc/ChangeLog:

	* config/arm/arm-mve-builtins-base.cc (expand): Initialize new_base.