Work around invalid register errors with 64-bit Windows (#28449)
Adding `-fno-asynchronous-unwind-tables` to the C compiler flags passed
to OpenBLAS works around errors claiming an invalid register for
`.seh_savexmm`. We've been running into this on the 64-bit Windows
buildbots.
See discussion in https://github.com/xianyi/OpenBLAS/issues/1708.
