Commit efbf08266b 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
1. Make _extext and _sidata coincide. Old _etext location did not include
.ARM.exidx sections, which were usually absent but not always. So flash
data was copied to RAM in wrong place.
2. Use decimal constants with "K" and "M" suffixes in .ld files instead
of hex constants, to make them easier to read and check for accuracy.