This PR is:

• a partial fix for gitian: GLIBC_2.29 not found error at bitcoind startup on bionic #21454 (see build: Avoid @GLIBC_2.29 libm symbols when --enable-glibc-back-compat #22281 for the complete fix of the bitcoind compatibility)
• a well documented alternative to the hackish build: Avoid fcntl64@GLIBC_2.28 symbol when --enable-glibc-back-compat #22287

As noted in #22244 (comment):

* `fcntl64` has a data reference, it's not ever called in the code (`objdump -d bitcoin-qt|grep fcntl64@plt` shows nothing)

but on 64-bit systems the only offender is libsqlite3.a.

From glibc docs:

• The fcntl function now have a Long File Support variant named fcntl64. It
  is added to fix some Linux Open File Description (OFD) locks usage on non
  LFS mode. As for others *64 functions, fcntl64 semantics are analogous with
  fcntl and LFS support is handled transparently. Also for Linux, the OFD
  locks act as a cancellation entrypoint.

From SQLite docs:

SQLITE_DISABLE_LFS

If this C-preprocessor macro is defined, large file support is disabled.

Due to peculiarities in glibc Large File Support implementation for 32-bit systems it looks safer to bump the minimum libc version to 2.28 for them.

Could be reverted back after bumping minimum glibc version to 2.28+.