Rollup merge of #137167 - martn3:reliable_f16_math-f16-erfc, r=tgross35

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

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

Author: Urgau