Recent toolchains support static and pie at the same time.

As with normal dynamic builds, allow --static to default to PIE
if supported by the toolchain.  Allow --enable/--disable-pie to
override the default.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
v2: Fix --disable-pie --static
v3: Update for QEMU_LDFLAGS.