Commit efbf08266b6 moved _estack in order to ensure 8-byte alignment
of the stack, but the address of _bootloader_dbl_tap must remain
right at the end of SRAM.
I verified by reading the source that the 4-byte-aligned address is
used for all samd21 / samd51 boards in
adafruit/circuitpython@efbf08266b. However, I only tested on
trinket_m0.
Closes: #739