Fixes stores to dcontext_t.whereami on AArch64 where a 64-bit register is written to a 32-bit field, overflowing it and clobbering the adjacent signals_pending field. This causes signals to not be delivered and fragments that were unlinked to not be relinked, raising an assert.
Tested on the proprietary application that reproduced the signal assert in every run: it is gone with this fix.
Fixes #4878 (closed)