stdio: Remove the usage of $(fno-unit-at-a-time) for siglist.c

stdio: Remove the usage of $(fno-unit-at-a-time) for siglist.c

The siglist.c is built with -fno-toplevel-reorder to avoid compiler
to reorder the compat assembly directives due an assembler
issue [1] (fixed on 2.39).

This patch removes the compiler flags by split the compat symbol
generation in two phases.  First the __sys_siglist and __sys_sigabbrev
without any compat symbol directive is preprocessed to generate an
assembly source code.  This generate assembly is then used as input
on a platform agnostic siglist.S which then creates the compat
definitions.  This prevents compiler to move any compat directive
prior the _sys_errlist definition itself.

Checked on a make check run-built-tests=no on all affected ABIs.

Reviewed-by: Fangrui Song <maskray@google.com>