cgen: reduce RAM usage, by avoiding a .str() call, for the final string builder, containing the final C program, used to write it to a file

[spytheman]

#0 22:33:59 ^ cgen_avoid_making_a_string_from_the_final_builder /v/oo>xtime ./vold -o x.c cmd/v
CPU: 2.56s      Real: 2.52s     Elapsed: 0:02.52        RAM: 428828KB   ./vold -o x.c cmd/v
#0 22:38:55 ^ cgen_avoid_making_a_string_from_the_final_builder /v/oo>xtime ./vnew -o x.c cmd/v
CPU: 2.54s      Real: 2.24s     Elapsed: 0:02.24        RAM: 422624KB   ./vnew -o x.c cmd/v
#0 22:39:01 ^ cgen_avoid_making_a_string_from_the_final_builder /v/oo>

The difference for V itself is ~6MB less, due to the avoidance of the final .str() creation.

_{Huly®: V_0.6-21660}