Fix log_backtrace assertion failure caused by LP64 fix.

Fix log_backtrace assertion failure caused by LP64 fix.

When libc.debug.malloc is enabled, stack backtraces fail with:

bionic/libc/bionic/libc_logging.cpp:378: assertion "conversion
    specifier unsupported" failed

The cause was a change to the format specifier from "%08x" to
"%0*x" to pass the field width as an additional parameter.
Unfortunately this modifier isn't supported by out_vformat() in
libc_logging.cpp.

Changed to use "%016x" if __LP64__ is defined; "%08x" otherwise.

Bug: 13177507
Change-Id: Ibf07263acf14da086d3c3788872f4b5477bd5708