contrib/bc: import version 6.5.0

This release that fixes an infinite loop bug in the (non-standard)
extended math library functions root() and cbrt(), fixes a bug with
BC_LINE_LENGTH=0, and adds the fib() function to the extended math
library to calculate Fibonacci numbers.

(cherry picked from commit 438a1101dc1f687928cdbe02cd7817a88a24f42f)

MFC after: 3 days