scripts/gen_compile_commands.py: adapt _LINE_PATTERN

For U-Boot's context, the regular expression defined by _LINE_PATTERN
should be adapted. Replace 'savedcmd' by 'cmd'.

Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>
Tested-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>