[gdb/testsuite] Fix build-ids in gdb.dwarf2/dwzbuildid.exp

[gdb/testsuite] Fix build-ids in gdb.dwarf2/dwzbuildid.exp

When looking at the execs from test-case gdb.dwarf2/dwzbuildid.exp using
readelf, I run into:
...
$ readelf -w dwzbuildid-ok > READELF
readelf: Warning: Corrupt debuglink section: .gnu_debugaltlink
readelf: Warning: Build-ID is too short (0x6 bytes)
...

Fix this by ensuring the Build-IDs are the required 20 bytes.

Tested on x86_64-linux.