When sanitizers report a problem around a global symbol, error report
contains symbol size. However, sizes of global symbols are 0 on
Darwin, because Dl_info doesn't have a field for symbol size.
This patch calculates the symbol size as the distance from given symbol
to next symbol in the same section, or to the end of section, if there
is no next symbol. The result is greater than or equal to the actual
size of symbol. Currently, only 64-bit Mach-O images are supported.