bugreport: add compiler info

bugreport: add compiler info

To help pinpoint the source of a regression, it is useful to know some
info about the compiler which the user's Git client was built with. By
adding a generic get_compiler_info() in 'compat/' we can choose which
relevant information to share per compiler; to get started, let's
demonstrate the version of glibc if the user built with 'gcc'.

Signed-off-by: Emily Shaffer <redacted>
Helped-by: Đoàn Trần Công Danh <redacted>
Helped-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>