BUG #17898: Build failure when strip is LLVM's version

The following bug has been logged on the website:

Bug reference:      17898
Logged by:          Ed Maste
Email address:      emaste@freebsd.org
PostgreSQL version: 13.10
Operating system:   FreeBSD
Description:        

In FreeBSD we are experimenting with the use of LLVM's binutils-like tools,
including installing llvm-objcopy as strip. When this is done PostgreSQL
fails to build, with error

====
STRIPPROG='strip -x' /bin/sh ../../../config/install-sh -c -m 644 -s libpq.a
'/wrkdirs/usr/ports/databases/postgresql13-client/work/stage/usr/local/lib/libpq.a'
strip: error:
'/wrkdirs/usr/ports/databases/postgresql13-client/work/stage/usr/local/lib/_inst.20299_':
not stripping symbol '.L.str' because it is named in a relocation
====

This occurs because llvm-objcopy is more strict, compared to GNU strip or
ELF Tool Chain strip. When -x is specified (i.e., discard all non-global
symbols) but the object to strip contains symbols that cannot be discarded
LLVM's strip emits an error. GNU and ELF Tool Chain strip instead just
retain the referenced non-global symbols.

Within the FreeBSD ports collection TCL and PostgreSQL are the only
significant software packages affected by this issue. TCL addressed this by
using --strip-unneeded instead of -x.

FreeBSD bug report:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270769
LLVM issue: https://github.com/llvm/llvm-project/issues/47468
TCL issue:
https://core.tcl-lang.org/tcl/tktview/718de2132f487cf29dfdd0be2b90fcc0107c2aa7