build: Provide LDFLAGS for pkg-config

Some pkg-config variants perform transformations on the --libs options.
This may lead to completely broken linker options.  Provide the LDFLAGS
as a variable.  Remove "Ldflags:" since this is an unsupported field.

Update #5169.

spec/build/bsps/pkgconfig.yml

@@ -13,6 +13,7 @@ content: |
   includedir=$${libdir}/include
 
   ABI_FLAGS=${ABI_FLAGS}
+  LDFLAGS=-B$${libdir} ${PKGCONFIG_LDFLAGS}
   RTEMS_ARCH=${ARCH}
   RTEMS_BSP=${BSP_NAME}
   RTEMS_BSP_BASE=${BSP_BASE}
@@ -25,7 +26,6 @@ content: |
   Version: ${RTEMS_VERSION}
   Description: RTEMS BSP ${ARCH}/${BSP_NAME}
   Cflags: $${ABI_FLAGS} -isystem$${includedir}
-  Ldflags: $${ABI_FLAGS} -B$${libdir} ${PKGCONFIG_LDFLAGS}
   Libs: $${ABI_FLAGS} -B$${libdir} ${PKGCONFIG_LDFLAGS}
 copyrights:
 - Copyright (C) 2020 embedded brains GmbH & Co. KG