unknown type name '__uint128_t'

[bob prohaska]

On an rpi3 with system at r344605, ports at 494124 and
CFLAGS=-O2 in /etc/make.conf, an attempt to compile 
www/chromium stopped, reporting

In file included from ../../base/third_party/libevent/event.c:49:
In file included from /usr/include/signal.h:42:
/usr/include/machine/ucontext.h:46:2: error: unknown type name '__uint128_t'
        __uint128_t     fp_q[32];
        ^
1 error generated.

followed a few lines later by

In file included from ../../base/third_party/libevent/evdns.c:37:
In file included from /usr/include/sys/types.h:46:
/usr/include/machine/endian.h:89:19: error: invalid operand in inline asm: 'rev16 ${0:w}, ${1:w}'
        __asm __volatile("rev16 %w0, %w1\n"
                         ^
/usr/include/machine/endian.h:89:19: error: invalid operand in inline asm: 'rev16 ${0:w}, ${1:w}'
/usr/include/machine/endian.h:89:19: error: unexpected token in operand
<inline asm>:1:8: note: instantiated into assembly here
        rev16 , 
              ^
In file included from ../../base/third_party/libevent/evdns.c:37:
In file included from /usr/include/sys/types.h:46:
/usr/include/machine/endian.h:89:19: error: invalid operand in inline asm: 'rev16 ${0:w}, ${1:w}'
        __asm __volatile("rev16 %w0, %w1\n"
                         ^
/usr/include/machine/endian.h:89:19: error: invalid operand in inline asm: 'rev16 ${0:w}, ${1:w}'

Errors continue in the same general vein until make gives up.

A month ago chromium would compile (with clang difficulties) and
even run more or less reasonably, so whatever is wrong happened
in a fairly recent timeframe.

Can anybody suggest a fix or workaround?

Thanks for reading,

bob prohaska