Build lib/msun tests with compiler builtins disabled

This forces the compiler to emit calls to libm functions, instead of
possibly substituting pre-calculated results at compile time, which
should help to actually test those functions.

Reviewed by: emaste, arichardson, ngie
Differential Revision: https://reviews.freebsd.org/D28577

(cherry picked from commit cf97d2a1dab8f2cddc4466fe64d37818339c73be)

riscv: Add a soft-float implementation of fabs()

We could just use a C implementation using __builtin_fabs(), but using
this assembly version guarantees that there is no additional prolog/epilog
code. Additionally, clang generates worse code for masking off the top bit
than GCC: https://bugs.llvm.org/show_bug.cgi?id=49377.

This fixes the RISCV64 softfloat world build after cf97d2a1dab8. That commit
added -fno-builtin to the msun tests which resulted in the first references to
fabs (previously the compiler inlined all calls).

Reviewed By: dim
Reported by: mjg
Differential Revision: https://reviews.freebsd.org/D28994

(cherry picked from commit 524b018d200408bed5eb0d2b892db5b9fb46808b)

riscv: Fix whitespace issues in fabs added in 524b018d2004

(cherry picked from commit 066dab17e7a4a78d43dbcef8119960ddc8090a73)