gdb/buildsym: Line record use a record flag

gdb/buildsym: Line record use a record flag

Currently when recording a line entry (with
buildsym_compunit::record_line), a boolean argument argument is used to
indicate that the is_stmt flag should be set for this particular record.
As a later commit will add support for new flags, instead of adding a
parameter to record_line for each possible flag, transform the current
is_stmt parameter into a enum flag.  This flags parameter will allow
greater flexibility in future commits.

This enum flags type is not propagated into the linetable_entry type as
this would require a lot of changes across the codebase for no practical
gain (it currently uses a bitfield where each interesting flag only
occupy 1 bit in the structure).

Tested on x86_64-linux, no regression observed.

Change-Id: I5d061fa67bdb34918742505ff983d37453839d6a