lib/msun: Exclude ignored-pragmas from -Werror

This avoids build failures due to the clang 12 warning:

'#pragma FENV_ACCESS' is not supported on this target - ignored

Clang 12 currently emits this warning for all non-x86 architectures.
While this can result in incorrect code generation (e.g. on AArch64 some
exceptions are not raised as expected), this is a pre-existing issue and
we should not fail the build due to this warning.

Reviewed By: dim, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29743

(cherry picked from commit 168234fa67c38f898b784b3265dd77ace0b0a2f9)