This adds a --toolchain CLI option to the script that can be either
  * gnu
  * llvm
  * chromium-llvm

The gnu and llvm options use the system's toolchain. chromium-llvm
uses Chromium's prebuilt LLVM toolchain with its Debian sysroot.

Previously, the script tried to combine the system's gcc with
libstdc++ from Chromium's Debian sysroot, which failed to find
glibc headers:

[1/4184] CXX obj/third_party/abseil-cpp/absl/base/base/cycleclock.o
FAILED: obj/third_party/abseil-cpp/absl/base/base/cycleclock.o
g++ -MMD -MF obj/third_party/abseil-cpp/absl/base/base/cycleclock.o.d -DUSE_UDEV -DUSE_AURA=1 -DUSE_GLIB=1 -DUSE_OZONE=1 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -D_GLIBCXX_ASSERTIONS=1 -DCR_SYSROOT_KEY=20250129T203412Z-1 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DABSL_ALLOCATOR_NOTHROW=1 -I.. -Igen -I../third_party/abseil-cpp -fno-strict-overflow -fno-ident -fno-strict-aliasing -fstack-protector -funwind-tables -fPIC -pipe -pthread -m64 -msse3 -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -O2 -fdata-sections -ffunction-sections -fno-math-errno -fno-omit-frame-pointer -g0 -fvisibility=hidden -Wno-unused-local-typedefs -Wno-maybe-uninitialized -Wno-deprecated-declarations -Wno-comments -Wno-packed-not-aligned -Wno-missing-field-initializers -Wno-unused-parameter -Wno-psabi -std=gnu++2a -Wno-changes-meaning -fno-exceptions --sysroot=../build/linux/debian_bullseye_amd64-sysroot -fvisibility-inlines-hidden -Wno-narrowing -Wno-class-memaccess -Wno-invalid-offsetof  -c ../third_party/abseil-cpp/absl/base/internal/cycleclock.cc -o obj/third_party/abseil-cpp/absl/base/base/cycleclock.o
In file included from /usr/include/c++/15/bits/version.h:51,
                 from /usr/include/c++/15/atomic:50,
                 from ../third_party/abseil-cpp/absl/base/internal/cycleclock.h:45,
                 from ../third_party/abseil-cpp/absl/base/internal/cycleclock.cc:23:
/usr/include/c++/15/x86_64-redhat-linux/bits/c++config.h:3:10: fatal error: bits/wordsize.h: No such file or directory
    3 | #include <bits/wordsize.h>
      |          ^~~~~~~~~~~~~~~~~