Revert "macros: Try to use the standard __func__ first in G_STRFUNC"

This reverts commit 9f75cc9e.

It breaks usage of G_STRFUNC when compiling applications with
`-Wpedantic` — `__func__` is not `#define`d, so G_STRFUNC was falling
through to using `__FUNCTION__`, which raises a warning with
`-Wpedantic`.

Fun times.

Fixes: #1755
10 jobs for backport-799-func-macro-glib-2-60 in 20 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
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passed #320480
cross-android_api21_arm64

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passed #320481
cross-android_api28_arm64

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cross-mingw64

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fedora-x86_64

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win32
msys2-mingw32

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win32
vs2017-x64

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cross-android_api21_arm64

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failed #320434
cross-android_api28_arm64

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fedora-x86_64

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  Coverage
passed #320438
coverage

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