dtlib: handle GCC linemarkers

A GCC linemarker of the form:

  # 1 "filename" 2 3 4

or so is not currently being handled, because the current regular
expression assumes the "flags" values (the numbers after "filename")
are limited to a single value. Tweak the regular expression to allow
for up to 4 flags, which is what GCC documents it may emit:

https://gcc.gnu.org/onlinedocs/gcc-10.2.0/cpp/Preprocessor-Output.html



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>