gdb: Fix occasional build failure

gdb: Fix occasional build failure

When running the configure check "checking for ELF support in BFD", LDFLAGS
were not being passed in to libtool. In OE/YP, we need these flags when using
uninative due to the games we play with the dynamic loader.

If a version of libzstd was built against a newer glibc, it would need
newer pthread symbols which it wouldn't find with the system linker. At
runtime this isn't an issue as it would be switched to use uninative but we
pass flags in LDFLAGS to allow this.

The bug is rare to reproduce as it depends on the host libzstd was built
against.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>