renamed gdb to gdb76
add gdb75 (7.5.1)
readd pkg/gdb: default to gdb75

instead of gdb76 until the bug described here
https://sourceware.org/bugzilla/show_bug.cgi?id=16370
is sorted out (fails to build without bash installed or mksh symlinked
to either /bin/sh or /bin/bash).
gdb76: add bash temporarily as dependency