Fix unused functions in invtrig_test.c

I only tested the WARNS=6 change on AArch64 and AMD64, but this file has
unused functions for architectures with LDBL_PREC == 53.

While touching this file change the LDBL_PREC == 53 checks to i386 checks.
The long double tests should only be disabled for i386 (due to the rather
odd rounding mode that it uses) not all architectures where long double
is the same as double.

PR: 205449
Fixes: 87d65c747a43 ("lib/msun: Allow building tests with WARNS=6")
Reported by: Jenkins