Fix build on Fedora and add instructions

[darkbasic]

I had to patch tools/gn/build/gen.py in order to fix this error:

[niko@localhost src]$ ./tools/gn/bootstrap/bootstrap.py --skip-generate-buildfiles -j$(nproc) -o out/Default/gn 
ninja: Entering directory `/home/niko/devel/ungoogled-chromium-ppc64le/build/src/out/Release/gn_build'
[1/1] LINK gn
FAILED: gn 
clang++ -O3 -fdata-sections -ffunction-sections -Wl,--gc-sections -Wl,-strip-all -static-libstdc++ -Wl,--as-needed -pthread -o gn -Wl,--start-group tools/gn/gn_main.o base.a gn_lib.a -Wl,--end-group -ldl
/usr/bin/ld: cannot find -lstdc++
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
Traceback (most recent call last):
  File "./tools/gn/bootstrap/bootstrap.py", line 142, in <module>
    sys.exit(main(sys.argv[1:]))
  File "./tools/gn/bootstrap/bootstrap.py", line 125, in main
    ['ninja', '-C', gn_build_dir, 'gn', '-w', 'dupbuild=err', '-j'+str(options.jobs)])
  File "/usr/lib64/python3.7/subprocess.py", line 347, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['ninja', '-C', '/home/niko/devel/ungoogled-chromium-ppc64le/build/src/out/Release/gn_build', 'gn', '-w', 'dupbuild=err', '-j32']' returned non-zero exit status 1.

[llebout]

Thanks, CI running: https://gitlab.com/lle-bout/ungoogled-chromium/-/jobs/280598117 Given that you tested it for Fedora, I'm just going to verify if it's still working on Ubuntu and then accept your changes.

Please note that if you think these changes also apply to upstream, please submit them upstream at https://github.com/Eloston/ungoogled-chromium

[llebout]

Worked fine. Note that I cherry-picked your commits manually into the tree, so I'll just close this here. Thanks!