build: Revert "Drop support for Ubuntu 18.04 Bionic"

https://chromium-review.googlesource.com/c/chromium/src/+/5598748

src/build/linux/sysroot_scripts/reversion_glibc.py

@@ -11,7 +11,7 @@
 # This constant comes from the oldest glibc version in
 # //chrome/installer/linux/debian/dist_package_versions.json and
 # //chrome/installer/linux/rpm/dist_package_provides.json
-MAX_ALLOWED_GLIBC_VERSION = [2, 28]
+MAX_ALLOWED_GLIBC_VERSION = [2, 26]
 
 VERSION_PATTERN = re.compile("GLIBC_([0-9\.]+)")
 SECTION_PATTERN = re.compile(r"^ *\[ *[0-9]+\] +(\S+) +\S+ + ([0-9a-f]+) .*$")

src/build/linux/sysroot_scripts/sysroot_creator.py

@@ -303,9 +303,17 @@ def hacks_and_patches(install_root: str, script_dir: str, arch: str) -> None:
         os.remove(qtchooser_conf)
 
     # __GLIBC_MINOR__ is used as a feature test macro. Replace it with the
-    # earliest supported version of glibc (2.28).
+    # earliest supported version of glibc (2.26).
     features_h = os.path.join(install_root, "usr", "include", "features.h")
-    replace_in_file(features_h, r"(#define\s+__GLIBC_MINOR__)", r"\1 28 //")
+    replace_in_file(features_h, r"(#define\s+__GLIBC_MINOR__)", r"\1 26 //")
+
+    # fcntl64() was introduced in glibc 2.28. Make sure to use fcntl() instead.
+    fcntl_h = os.path.join(install_root, "usr", "include", "fcntl.h")
+    replace_in_file(
+        fcntl_h,
+        r"#ifndef __USE_FILE_OFFSET64(\nextern int fcntl)",
+        r"#if 1\1",
+    )
 
     # Do not use pthread_cond_clockwait as it was introduced in glibc 2.30.
     cppconfig_h = os.path.join(