Whenever a call to __chkstk was made, the frame lowering previously
omitted the aligning (as NumBytes was reset to zero before doing
alignment).
This fixes https://github.com/llvm/llvm-project/issues/56182.
The initial version of this produced invalid code for small
functions with no local stack allocations, if those functions
were marked with the "stackrealign" attribute. If building
with -mstack-alignment=16 (which otherwise mostly would be a
no-op), this attribute is added on the main function.
Differential Revision: https://reviews.llvm.org/D135687
