build: Provide LDFLAGS for pkg-config

Some pkg-config variants perform transformations on the --libs options.
This may lead to completely broken linker options:

https://github.com/pkgconf/pkgconf/issues/371

Provide the LDFLAGS as a variable.  Do not add the ABI_FLAGS to LDFLAGS
since they are already contained in the CFLAGS.  The linker command line
should include the compiler flags so that link-time optimization works
properly.

Update #5165.

spec/build/bsps/pkgconfig.yml

@@ -13,6 +13,7 @@ content: |
   includedir=$${libdir}/include
 
   ABI_FLAGS=${ABI_FLAGS}
+  LDFLAGS=-B$${libdir} ${PKGCONFIG_LDFLAGS}
   RTEMS_ARCH=${ARCH}
   RTEMS_BSP=${BSP_NAME}
   RTEMS_BSP_BASE=${BSP_BASE}