cortex-m-rt v0.7.7

• Align the default _stext to the alignment required by the .text section. Fixes
  the lld warning address (...) of section .text is not a multiple of alignment (...).
  • Note: _stext is not auto-aligned if it's explicitly overridden by the user. It's their responsibility to align it, else the warning will show.
• Update to syn v3.