build: use -Wl,--gc-sections when appropriate #2469

Job	Run time
spelling	13s
Generate build matrix	11s
validate commits	0s
python linting	28s
bookworm - test-install	12m 18s
bookworm - 32 bit	13m 1s
noble - test-install	13m 18s
el9 - test-install	12m 17s
alpine - test-install	12m 25s
fedora40 - test-install	12m 17s
jammy - test-install	12m 0s
focal - py3.8	17m 34s
bookworm - gcc-12,content-s3,distcheck	19m 39s
fedora40 - clang-18	16m 32s
coverage	23m 54s
el8 - ascii	19m 41s
el8 - test-install	12m 54s
el8 - system,coverage	7m 45s
inception	21m 51s
flux-accounting check	4m 46s
flux-sched check	7m 4s
flux-pmix check	13m 2s
flux-pam check	4m 21s
Generate docker manifest	1s
	4h 17m 32s

Provide feedback