[Bug debug/93751] -g1 does not behave per manual

[stilor at att dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93751

--- Comment #11 from Alexey Neyman <stilor at att dot net> ---
I decided to name the option `-gexternal-variables` - it seems more in-line
with the existing options (which omit "dwarf" from the option name even if the
option is only relevant to DWARF debugging info).

Also, I decided against having a negatable option (`-gno-external-variables`)
as it would lead to funny results with `-g2` or `-g3` (where we'd generate DIEs
for local and static variables, but not for external ones).