This bug was closed following the resolution of #50058

However, I still can't build on musl due to seccomp-related issue.

In the original message, I forgot to attach actual output from GCC; here
it goes:

x86_64-gentoo-linux-musl-gcc -fno-common -Wall -Warith-conversion
-Wdate-time -Wdisabled-optimization -Wdouble-promotion -Wduplicated-cond
-Wextra -Wformat-signedness -Winit-self -Winvalid-pch -Wlogical-op
-Wmissing-declarations -Wmissing-include-dirs -Wmissing-prototypes
-Wnested-externs -Wnull-dereference -Wold-style-definition -Wopenmp-simd
-Wpacked -Wpointer-arith -Wstrict-prototypes -Wsuggest-attribute=format
-Wsuggest-attribute=noreturn -Wsuggest-final-methods
-Wsuggest-final-types -Wtrampolines -Wuninitialized -Wunknown-pragmas
-Wunused-macros -Wvariadic-macros -Wvector-operation-performance
-Wwrite-strings -Warray-bounds=2 -Wattribute-alias=2 -Wformat=2
-Wformat-truncation=2 -Wimplicit-fallthrough=5 -Wshift-overflow=2
-Wvla-larger-than=4031 -Wredundant-decls -Wno-missing-field-initializers
-Wno-override-init -Wno-sign-compare -Wno-type-limits
-Wno-unused-parameter -Wno-format-nonliteral -I. -I../src -I../lib
-I. -I./../src -I./../lib -Wl,-O1 -Wl,--as-needed -march=native -O2
-pipe seccomp-filter.c \ -lseccomp -o seccomp-filter
seccomp-filter.c: In function 'export_filter':
seccomp-filter.c:134:12: warning: implicit declaration of function 'TEMP_FAILURE_RETRY' [-Wimplicit-function-declaration]
  134 |   int fd = TEMP_FAILURE_RETRY (
      |            ^~~~~~~~~~~~~~~~~~
seccomp-filter.c:134:12: warning: nested extern declaration of 'TEMP_FAILURE_RETRY' [-Wnested-externs]
/usr/lib/gcc/x86_64-gentoo-linux-musl/10.3.0/../../../../x86_64-gentoo-linux-musl/bin/ld: /var/tmp/portage/app-editors/emacs-28.0.9999/temp/ccIiDmil.o: in function `export_filter':
seccomp-filter.c:(.text+0x199): undefined reference to `TEMP_FAILURE_RETRY'
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:432: seccomp-filter] Error 1
make[1]: Leaving directory '/var/tmp/portage/app-editors/emacs-28.0.9999/work/emacs/lib-src'
make: *** [Makefile:430: lib-src] Error 2