Beaglebone black console output : (...) {standard input}: Assembler messages: {standard input}: Warning: end of file not at end of a line; newline inserted {standard input}:458: Error: ARM register expected -- `str r3,[r' armv7a-hardfloat-linux-gnueabi-gcc: internal compiler error: Killed (program cc1) Please submit a full bug report, (...) Reproducible: Always
Created attachment 388606 [details] build.log
Created attachment 388608 [details] emerge --info
Given the messages before compiler getting killed, I'd go with "ran of of disk space/memory".
You ran out of memory. You could remove -pipe from C*FLAGS but in the end cross-compiling (or adding RAM) would be the best cause.