tests: Also gate f16::erfc() doctest with reliable_f16_math cfg

[martn3]

In #136324 the doctest for f16::erf() was gated with reliable_f16_math. Add the same gate on f16::erfc() to avoid:

rust_out.71e2e529d20ea47d-cgu.0:\
(.text._ZN8rust_out4main43_doctest_main_library_std_src_f16_rs_1321_017h485f3ffe6bf2a981E+0x38): \
undefined reference to `__gnu_h2f_ieee'

on MIPS (and maybe other architectures).

r? tgross35