[lldb] Fix m_hwp_regs size for ppc64le (PR54520)

The size of the m_hwp_regs array should match the default value of
m_max_hwp_supported. This ensures that no out-of-bounds accesses
occur, even if the array is accessed prior to a call to
ReadHardwareDebugInfo().

Fixes https://github.com/llvm/llvm-project/issues/54520, see also
there for additional background.

Differential Revision: https://reviews.llvm.org/D136144