build: Fix linker path

Use -B for the linker since some architectures use extra start files defined by
the GCC specification and provided by the BSP, for example powerpc.  Start
files are not found by -L.

In the long run, the GCC defined start files should be provided by GCC.  This
is a GCC 11 topic.

Update #3818

spec/build/bsps/pkgconfig.yml

@@ -23,8 +23,8 @@ content: |
   Version: ${RTEMS_VERSION}
   Description: RTEMS BSP ${ARCH}/${BSP_NAME}
   Cflags: $${ABI_FLAGS} -I$${includedir}
-  Ldflags: -L$${libdir} ${PKGCONFIG_LDFLAGS}
-  Libs: -L$${libdir} ${PKGCONFIG_LDFLAGS}
+  Ldflags: -B$${libdir} ${PKGCONFIG_LDFLAGS}
+  Libs: -B$${libdir} ${PKGCONFIG_LDFLAGS}
 copyrights:
 - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
 enabled-by: true