gdb: use actual DWARF version in compunit's debugformat field

gdb: use actual DWARF version in compunit's debugformat field

The "info source" command, with a DWARF-compile program, always show
that the debug info is "DWARF 2":

    (gdb) info source
    Current source file is test.c
    Compilation directory is /home/smarchi/build/binutils-gdb/gdb
    Located in /home/smarchi/build/binutils-gdb/gdb/test.c
    Contains 2 lines.
    Source language is c.
    Producer is GNU C17 9.3.0 -mtune=generic -march=x86-64 -g3 -gdwarf-5 -O0 -fasynchronous-unwind-tables -fstack-protector-strong -fstack-clash-protection -fcf-protection.
    Compiled with DWARF 2 debugging format.
    Includes preprocessor macro info.

Change it to display the actual DWARF version:

    (gdb) info source
    Current source file is test.c
    Compilation directory is /home/smarchi/build/binutils-gdb/gdb
    Located in /home/smarchi/build/binutils-gdb/gdb/test.c
    Contains 2 lines.
    Source language is c.
    Producer is GNU C17 9.3.0 -mtune=generic -march=x86-64 -g3 -gdwarf-5 -O0 -fasynchronous-unwind-tables -fstack-protector-strong -fstack-clash-protection -fcf-protection.
    Compiled with DWARF 5 debugging format.
    Includes preprocessor macro info.

The comp_unit_head::version field is guaranteed to be between 2 and 5,
thanks to the check in read_comp_unit_head.  So we can still use static
strings to pass to record_debugformat, and keep it efficient.

In the future, when somebody will update GDB to support DWARF 6, they'll
hit this assert and have to update this code.

Change-Id: I3270b7ebf5e9a17b4215405bd2e365662a4d6172