Document one more skia armv7 build caveat

Sadly I could not find a reliable way of retrieving this from within
CMake (as for example CMAKE_CXX_FLAGS is already folded into
CMAKE_CXX_COMPILER_ARG1 by the the time our cmake code runs), so
document this as a workaround for now.

docs/reference/src/advanced/backends_and_renderers.md

@@ -98,3 +98,20 @@ issues we're aware of and how to resolve them.
   The error happens when that path contains spaces. By default that's in `%HOMEPATH%\.cargo`,
   which contains spaces if the login name contains spaces. To resolve this issue, set the `CARGO_HOME`
   environment variable to a path without spaces, such as `c:\cargo_home`.
+
+* Compilation error when compiling for ARMv7 with hardware floating-pointer support
+
+  You may see compiler errors that contain this message:
+
+  ```
+   Unable to generate bindings: ClangDiagnostic("/home/runner/work/slint/yocto-sdk/sysroots/cortexa15t2hf-neon-poky-linux-gnueabi/usr/include/gnu/stubs-32.h:7:11: fatal error: 'gnu/stubs-soft.h' file not found\n")
+  ```
+
+  The Skia build invokes clang in multiple occasions and is sensitive to compiler flags
+  that affect the floating point abi (such as `-mfloat-abi=hard`), as they affect header file lookups.
+
+  The solve this, set the `BINDGEN_EXTRA_CLANG_ARGS` environment variable to contain the same
+  flags that your build environment also passes to the C++ compiler.
+  
+  For example, if you're building against a Yocto SDK, then you can find these flags in the
+  `OECORE_TUNE_CCARGS` environment variable.