"This is not a register operand!" compiling firefox on RPI2

[bob prohaska]

To date all attempts to compile the firefox port terminate
with an error message along the lines of 
....
/usr/ports/www/firefox-esr/work/firefox-45.8.0esr/media/webrtc/trunk/webrtc/base/basictypes.h:97:9: note: 
      previous definition is here
#define ARCH_CPU_LITTLE_ENDIAN
        ^
Assertion failed: (isReg() && "This is not a register operand!"), function getReg, file /usr/src/contrib/llvm/include/llvm/MC/MCInst.h, line 64.
c++: error: unable to execute command: Abort trap (core dumped)
c++: error: clang frontend command failed due to signal (use -v to see invocation)
FreeBSD clang version 4.0.0 (tags/RELEASE_400/final 297347) (based on LLVM 4.0.0)
....

This is on -current, up to date as of March 24, 2017.  The error message 
is roughly the same for firefox, firefox-esr and chromium.

Thanks for reading,

bob prohaska